What Causes Swelling Inthe Legs After Physical Excertion?

My husband s legs are swelling with activity, and over the last few months he has gone form being very active and strong, to tiring easily and feeling very weak. His doctor has sent him for evaluation by physical medicine, neurology, vascular medicine and run numerous xrays and blood tests. Unfortunately, they are unable to identify a cause of the problem. We feel we need to seek additional help but are not sure where to begin. Any recommendations?