Well, the vet has found nothing, right? Is there something in your home that makes him nervous or stressed out?
I have cats, all rescued. Sable was a rescue from a abusive situation and the only cat I have who has been declawed. The other cats can be mean to her. I keep it at a minimum as much as I can. But, if the others have chased her more in one day, she will throw up because of being afraid.
Generally I will call her to me to sooth her which quells the up-chuck most of the time.
Or if he is an indoor only cat, he may want to be indoor/outdoor. I have done that for only my adult cats who have been in for six years. And surprisingly they pretty much stay in our fenced in back yard. Both are very happy and are losing weight, fat the vet said they don't need.
Get creative and look at non-traditional methods to remedy the problem. Good luck
