I would go to the GP and get a referral to an ENT specialist to rule out physical causes.

I personally have a sensitive gag reflex. It got a lot better when I was in late childhood, early teens as I was able to work out volume of bites more easily. However, I am 34 now and still throw up if I misjudge my bite size. It hasn't happened for a few months but I am always conscious of it.

Oh, texture also triggers my gag reflex. I can guarantee to puke if I eat peas or beans. I need to prepare myself to eat bananas and Weetbix. Have you noticed what food generally causes him to gag initially before the vomit?