Diabetes type 2 is the most common cause of wounds not healing well. And--high blood sugar can go undiagnosed for a long time. When one is ill--it rears its ugly head.
For instance, my blood sugars stay pretty normal-unless I'm sick or hurt--then they skyrocket.
If that's been ruled out--I'm not sure of what else is going on, but circulation issues can also hinder healing. Some with CRPS don't heal very quickly either.
Is there a loss of hair on his legs by chance? (a common finding in circulation issues)