this is a far shot but I have seen vomiting as a possible manifestation of epilepsy. I would say, get him a sleep test.
The best thing you could do right now is keep a daily log of everything...yes EVERYTHING he eats, drinks, and when he wakes/sleeps and when he vomits...etc. Make sure you are very detailed. This helped in diagnosing my son. Although his was GERD, I think this might be the only way to give the doctors some starting point.
Does he have any new friends? Anybody's house he goes over to that he didn't before? Does he crave anything weird, like eat cardboard, ice, soap...etc. I know that is a wierd one but sometimes when there is a hormonal/chemical imbalance there are ways the body tells you, sometimes by eating things like sand, dirt, soap...
Please keep us posted. I hope they figure it out soon. Just remember log, log, log!