One thing I can suggest is an improvment in his diet. Boost fresh veggie content dramatically, and drink lots of water. It'll help with his weight, and may even help with his diabetes.
As for the antibiotics, such prolonged exposure may be keeping whatever infection is in his leg in check, but is likely hampering the bodies natural bacterias. Look into acidpholus (sp) suppliments.
It's good that he quit smoking. Does he do any breathing exercises? The best one I know is to inhale as deep as you can thru your nose, hold it for ten seconds, then exhale slowly thru your mouth.
Finally, regarding the leg itself, how much exercise does he get on it? Increasing blood flow dramatically to the region can help with the healing process. I don't know how difficult walking is for him, but some 'fast hard walking' is one of the best exercises for the legs. I mean swinging the arms and really getting the legs going. This particular exercise will start building new blood vessels in his legs immediately, but will take about a month to see any noticable effect. But this is truly worth it, esp. if the alternative is chopping off his leg! At first he may not be able to do any great distance, but every day he will gain a little more stamina.
All these things will require a lifestyle change, but if he values the quality of his life, these are things he should consider. It *will* be painful (well, except the eating better part, although cutting down bad foods can be difficult) but the results far outweigh the alternative.