He sounds very uncomfortable. Dogs that are painful will hesitate jumping or climbing stairs and display the sort of behavior he is, either because it hurts to do the activity or because they feel weaker than usual.
Some dogs are prone to a problem with their intervertebral discs, which are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture and place pressure upon the spinal cord which can lead to pain, and in severe cases paralysis.
Radiographs can sometimes be diagnostic but often early on in the disease process, because the discs are soft tissue not bone, everything will look normal. An MRI is the best way of diagnosing disc disease.
If the dog is painful but has no evidence of paralysis we can try strict rest, anti-inflammatories and pain medications for several weeks to allow healing.
If there is evidence or weakness or paralysis then surgery by a board certified veterinary neurologist, as soon as possible, is indicated.
Ideally he should see his veterinarian. If this is indeed a disc problem your veterinarian can prescribe a steroid or nonsteroidal anti-inflammatory to relieve pressure on his spinal cord and nerve roots, as well as something for pain too, such as Tramadol. And if he is having painful muscle spasms then a muscle relaxant such as methocarbamol as well.
Make sure to tell your veterinarian that you have given aspirin. Aspirin does cause stomach and intestinal irritation and ulceration as well as clotting problems so should not be given for more than 2 to 3 days consecutively and should always be given with a meal. You should watch for lack of appetite, vomiting, blood in the stools or dark tarry stools and stop immediately if you see those. Do not use aspirin if your dog has liver or kidney disease or a history of a sensitive stomach or clotting problems.
Be aware because you chose to use aspirin your veterinarian will be limited on what they can give initially as there must be a 5 to 7 day washout period between different nonsteroidals or nonsteroidals and steroids. If this washout period is not observed then side effects such as bleeding gastrointestinal ulcers and organ damage could occur.
He should be closely confined starting now. No stairs, running or jumping. If you have a crate for him I highly recommend using it. The less he moves around the more comfortable he will be and the faster he will heal.
He should go out on a leash to relieve himself. Do not use a collar for him, a harness with a clip on the harness at the front of his chest to attach the leash more evenly distributes forces. This is much better if he pulls on his leash. You will need to confine him for several weeks, even as he starts to feel better or he may reinjure himself.
Keeping him on the thin side is recommended to decrease stress on his back, but is no guarantee that he won't have another episode. Once a dog has one bad disc the likelihood of another is very high.
If you are interested in reading more here is a link to an excellent article about intervertebral disc disease, its causes and therapy: http://www.petwave.com/Dogs/Dog-Health-Center/Bone-Joint-Muscle-Disorders/Intervertebral-Disk-Disease/Symptoms.aspx
There are other less common causes of neck pain such as infections, tumors of the vertebrae or the spinal cord itself or fibrocartilagenous emboli but far and away disc disease is the most common cause of back pain in dogs.
Because he is a young large breed fellow we also must consider other common problems in these puppies, including elbow dysplasia, eosinophillic panosteitis, or shoulder OCD (osteochondrosis dessicans). These all worsen with exercise or excess use. They are diagnosed by radiographs most of the time but sometimes require an MRI to diagnose.
Because his discomfort is not improving I highly recommend that he see his veterinarian after exercise or play when his limp will be more apparent. They will examine him to look for the painful area and will likely want to take radiographs under sedation. Treatment will depend upon diagnosis but can include rest and anti-inflammatories, physical therapy, and omega 3 fatty acids and glucosamine chondroitin supplements to possible surgery to remove cartilage flaps depending upon the problem.
In the meantime I do recommend strict rest. Soft tissue injuries can take months of rest to heal and won't heal if he continues to return to full activity too soon.
I also recommend glucosamine/chondroitin products (like cosequin-ds) plus omega-3 fatty acids. Adding an omega 3 which is a natural anti-inflammatory will help as together they work synergistically, better than either one alone. These supplements can take 6 to 8 weeks to see an improvement with their use.
Please let me know if you have any further questions.