It could definitely be psychological like bunny suggested. My dh had that issue when we first got together and he was only 22-23. He also is quite big at about 260-270 lbs and not very tall, maybe 5'5" -5'6".. so in his case, weight probably plays a role too. When we first got together and started getting intimate I think he was just kinda shy and self conscious and had a hard time getting "ready" and staying "ready" in your case it could be a combination of the medication and that the atmosphere of making love has changed, maybe he feels nervous or pressured to perform to participate in the sway now. As well as speaking with his doctor to see if the medication could be causing this, you might also try to talk to him to see if he is feeling any psychological distress in the bedroom as far as nerves or pressure to perform. My dh and I talked about past sexual experiences that he had had that left a negative impact on him and I always am complimenting him on his body and he hasn't had as much as an issue anymore although sometimes he has days where it just won't cooperate, most days are fine now.
