I truly commend your desire to help your boyfriend, I think that is a beautiful and noble thing. His paralysis from the chest down does present quite a few issues. You are right to ask about them so you know what might be and what to possibly expect.
First and foremost, inability to move and shift position can and will lead to pressure ulcers. This happens because there is little blood flow to the area that has the unending weight upon it, so the tissue essentially dies. Preventing pressure sores is easy; its moving and repositioning him. The areas of most risk for him are his buttocks and backs of his legs/ feet. If he has full use of his arms, he can use his arms to shift his weight in the chair and help avoid some of this, but he will need help in keeping mobile so he avoids this very preventable and painful malady.
How is he urinating and defecating? Does he have to be catheterized? Is he incontinent? Does he have an indwelling catheter and leg bag? Finding out how he urinates is important. He may have an indwelling catheter that is surgically implanted and drains from bladder itself. This is called a supra pubic catheter and care for it involves draining the urine from the bag and keeping the stoma...the area where its attached to his skin...clean and dry. The catheter itself also needs changing about once a month. Information on suprapubic catheters is here: http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000145.htm
He may have a condom catheter. This is like a condom, but has a catheter at its tip. These can be left on for a day and then replaced with a new one, but of course the bag has to be emptied and the penis checked for signs of skin breakdown. Information about catheters is here: http://www.nlm.nih.gov/medlineplus/ency/article/003981.htm The medical term for bladder disorders due to nerve disorders is called neurogenic bladder, and more information is available about this here: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001761/ . Your boyfriend is also at somewhat higher risk for urinary tract infections because of the damage to the nerves that control the bladder; he can not likely empty his bladder as uninjured people do. Here is information about urinary tract infections that is specific to those with indwelling catheters: http://www.nlm.nih.gov/medlineplus/ency/article/000483.htm
Defecation is an issue as well. Being paralyzed, it is likely that he has extreme constipation. He could get a fecal impaction as a result (where the feces become unable to be passed) and that can be a medical emergency as potentially intestines could rupture. He may be on medications to soften his stools. Constipation can also cause autonomic dysreflexia, discussed below.
The major concern about urination and defecation is an issue called autonomic dysreflexia. It is a medical emergency should it happen, as it can lead to strokes, and it can be caused in persons with nerve spinal damage by bowel and bladder issues such as constipation, a urinary bag that is not drained, many others. What happens here is that the autonomic reflex system, the one that controls involuntary functions like breathing, heart beat, blood pressure get over stimulated. Symptoms are high blood pressure, a severe headache, sweating, low pulse. Treatment is geared at stopping whatever is causing the irritation, whether its a urinary tract infection, a skin irritation, hemorids, pressure sores, or even clothing that is too tight. OF course if you suspect this, please dial 911. More information on this is here: http://www.christopherreeve.org/site/c.mtKZKgMWKwG/b.4453413/k.5E2A/Autonomic_Dysreflexia.htm
Some people with spinal cord damage have a hard time regulating their internal and external body temperatures. He may be cold, or hot, or have fever and not even realize it. So, you can look for signs that he is overheated such as sweating, or flushed skin.
Because he can not move his legs (I am presuming he has control of his arms if his paralysis is chest down) he may need to keep his feet in braces of some sort. This is because when people can not use a limb, it tends to contract. Perhaps you've noticed that when you relax your hand, your fingers and wrist curls. BUT, if you have lost the use of that hand, the tendons in that hand will lose elasticity and become short. And over time the hand will curl more and more and become completely contracted. In fact, this condition is called contractures. A very similar thing can happen in the legs (but was easier to discuss using the hand as example). This can be prevented by range of motion exercises which I am sure your boyfriend hates but are important. They involve moving his legs/ ankles/ feet/ toes in all directions that the limb is designed to function in.
He will have a higher risk of fractures than other people. Paralysis and immobility causes the bones to leech calcium, they become brittle and its much like osteoporosis. Care should be taken that nothing bangs into his legs as something that would cause a bruise in someone else may cause a break in him. I have seen people (children) with paralysis have devices called "standers". What they do is allow him to stand up, and place weight on his legs andhelp decrease somewhat the chances of osteoporotic changes due to immobility.
I am sure that when you and he are ready you will want and active and healthy sex life. That is very possible, just be aware that some accommodations have to be made. If he is paralyzed from the chest down, I highly doubt that he can get an erection, and he may feel horrible about that; he may need reassurance that it is OK and you accept and love him as he is. There are plenty of ways he can make you feel good, using arms, and his mouth if he likes oral sex. The positioning will be on you though, so he can access you in the way thats easiest for him. Ask him whats easy and what is difficult. I am sure he will appreciate the open and honest upfront conversation. I have not read this book myself, but I understand it has gotten great reviews from many who are disabled. It is called The Ultimate Guide to Sex and Disability, For All of us Who Live With Disabilities, Pain, and Chronic Illness and the amazon.com link is here: http://www.amazon.com/Ultimate-Guide-Sex-Disability-Disabilities/dp/1573441767
I realize writing this that it sounds like quite a lot. But your boyfriend has been caring for himself with this condition for long time. He is likely to be expert in his own care, and may be happy to have your help. And I am betting that he will be flattered at your interests and desire to help him. However he may want privacy in some things, and that is OK also; let him tell you his needs. I know many feel its bad to question someone with a disability, they mistakenly believe it could be seen as rude or insensitive. But asking him how best to maintain his health shows him you have an active interest, and he will likely appreciate not being treated as a fragile and breakable object.
I hope I was able to help. Please only rate my service to you when you are satisfied with my answer; thats how I get paid here. If you have any further questions I am more than willing to work with you further. As I wrote before I think you are quite wonderful to be asking this question.