What you are describing is something called ataxia. This can have many causes, and it generally comes in three different varieties. From what you are describing, it sounds like your dog is having a proprioceptive disorder. This is usually caused by a problem with the spinal cord. This can be from infection, inflammation, spinal injury, intervertebral disk disease, or cancer. The most common...is something called intervertebral disk disease.
Basically, what happens is that one or more of the cushions (disks) inbetween the dogs vertebrae has protruded into the spinal canal and is impinging on the spinal cord. This can cause signs that are mild and limited to pain, or it can be severe enough to cause complete paralysis and loss of sensation in the limbs. Time is of the essence with treating this disease, so he should be taken to an emergency clinic immediately to be evaluated and to be started on steroids to stop the inflammation in his spinal cord. In some dogs, steroids for several weeks and very strict cage rest can stop the progression and return the dog to normal (or close to normal) function. In other cases, surgery is necessary to correct the problem. This is usually done by a specialist in veterinary surgery at a referral center or college of veterinary medicine.
I am attaching some client information handouts that I use in my practice for you to read. One is about the different causes of ataxia, and the other is about IVD. I hope that you find them useful.
Click HereClick Here 2
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.