The sudden collaspe 4 months ago, anorexia, ataxia, and pale gums (anemia) are all pointing to cancer, and possibly hemangiosarcoma. The anemia may be causing the weakness. Since this has been going on for awhile it may be at a terminal stage.
You should first find another Vet. Regardless of cause it would be impossible for his blood work to be "normal". Have an ultrasound run on your dog. If there are any tumors internally this diagnostic should find them. The best case senario would be that a single tumor (e.g. spleen) may be surgically removed. The worst case is that the cancer has spread, and you will have to make a difficult decision.