Take your son to see a pediatric gastroenterologist.
Has he had any tests for sugar levels ,as he has some weight yet eats healthily and exercises maybe something going on in that direction,he throws up as he is sleeping, and doesnt recollect throwing up the next day, as you saying baffling.Something is causing it I would persist with the Doctor to have some tests run ...good luck
Our pediatrician does not believe it is an interaction of the meds. He does not eat anything after 6pm. We are a very active, healthy-eating family, which is why we are so baffled. He made straight A's in school and played one sport per season. He has lots of friends, and great conduct grades. He does get up to go to the bathroom once or twice per night, and only sleeps about 9 hours each night.
Well I would certainly ask why this is happening, could it be the meds have you checked if they interact with each other .What time does he have his last meal, perhaps see if it helps that he doesnt eat , only drinks water after 6pm or earlier depending on the evening meal time. You say he is heavier so is he eating healthily and exerciasing .How does he do at school ,has he friends ,get on with his teacher ...?