I'm glad to hear that the vet found out and he's got some medicine for it. If he gets the same kind as our cat and you notice he suddenly get fatter, don't worry! It can be the side effect of the medicine, he will loose it again when he stops taking it.
My friend who is a nurse told us the reason: Our cat got Prednosolon for 10 days, which contains Binyrebarkhormon and that's the reason. He is back to normal again!