What Could Constant Swelling Of Back, Legs And Feet Suggest?

About three months ago my 32 year old son thought he had hurt his back exercising...he was in a lot of pain, went to his doctor who did x-rays, blood tests, etc. and told him he could find nothing but sent him to a physical therapist. The pain has traveled down his back, to his legs and now to his feet which are swollen. He wakes up "sore" and while his back and legs appear to be somewhat better, his feet are still swollen. Tests indicate that he does not have arthritis or Lymes Disease. He takes Motrin morning and evening. He continues to work but is in constant discomfort. He has never had any physical problems until now. He is 6'2" and weighs around 180-185. He does the exercises the doctor/physical therapist suggest but does not seem to be better.