oh dear, I do feel for you, you must be out of your mind with worry. As Kita says, it can be any number of things and I dont think anyone on here is best suited to diagnose, or even suggest what it may be.
I hope the vet has given you something to help in the interim for when he has these fits or at least told you what to do, I think the only thing is to comfort him, but you need to know what to look for as it may be that you would have to call the vet in an emergency.
You say he hasnt eaten anything, but is there a chance he has picked anything up, or eaten a poisonous plant? I am wondering whether blood tests may not show anything up and he may need a scan to determine anything untoward with his brain - but I hope you find out soon, and he is ok.