Hello, I'm sorry to hear about your dog.
It sounds like there is definitely something going on here that needs to be addressed. The roundworms are more likely due to an underlying immunosuppression rather than a primary problem. Roundworm infections are not all that common in older dogs as they can mount a response and clear them without medication. The fact that this 8 year old dog did not clear a roundworm infection without meds is a big red flag that the immune system is not functioning properly. Now we need to find out why.
I would start by running full bloodwork on him. This will test organ function, electrolytes, calcium: phosphorous balance, white cell and red cell counts. I would imagine that this will help narrow down the long differential list. More specific tests for endocrine function may be needed if Addison's disease is suspected. This is where the adrenal glands do not function properly.
The spinal pain is a little odd and may be an incidental finding here. Unless there is cancer that has spread to the spine, it's hard to tie in neck pain with immune system suppression and the rest of the GI signs. Doing a good neuro exam will help determine if the spinal pain is actually neurologic or if it's orthopedic in origin.
Obviously there is quite a bit that needs to be done here, but a good thorough exam as well as screening bloodwork should narrow the list down and hopefully diagnose something that can be treated or at least managed to make your dog more comfortable and get him back on the road to recovery.
I hope this helps, let me know if you have any other questions.
Judging from what you understood, what would be your most honest opinion? Should I put my dog down? Would putting him to sleep be the best decision? What are the chances, just from making an eduacated guess as to what my dog has, of him making a full recovery? (percentage wise) Do you think he will suffer more?
I would never want to do this to him, but I would much rather remember him for being one of the best and energetic dogs I have ha. I dont want to make him suffer. What would you recommend?
Is there something I can buy at least for his wound that you may recommend? Anything I should avoid giving him at all costs? I would truly appreciate your honesty. Should I avoid giving him massages? Should I introduce a source of food that would help him, something light like...?
It's hard to say what's causing this. I would run some diagnostics before recommending euthanasia. If this is due to something that can be managed or treated then it's worth treating. I think the prognosis is guarded given the signs and his age/ breed but I wouldn't write off a full recovery quite yet. I think a 50-60% chance of it being treatable would be appropriate just based on the history and of course this could change drastically based on exam findings/ blood work findings.
As for the wound I would probably just clip the hair around the wound and then clean it with a dilute hand soap solution. You can then put on a topical antibiotic like Neosporin and wrap it for 1-2 days to keep him from licking at it.
As for food, you may want to try cooking a chicken/ rice or hamburger/ rice and see if he'll eat that for you. If he does then you can mix in some of his regular food and see if you can get him back on his normal diet.
I don't think massages are helpful at this point as he appears to be a little painful from the sounds of it.