I can relate to this.  Our now 7 year old SWCAH daughter was the same at your son’s age.  She had a very sensitive gag reflex as well and didn’t tolerate chunky food.   After a while we kind of narrowed her vomitting down to potatoes of all things.  Any type of potato just didn’t seem to agree with her.  We avoided potatoes for a while and now she’s fine.  She doesn’t gag easily on food any more either.