Everyone is different, usually i find my joints ache and a fatigue then come the tummy upset can be anything from an hour to 3 hours. Try lactose free milk with cereal. All the best with finding right diet. Dietician visit might help too. Gluten is in lots things, toothpast etc.
Definitely research Adult GI. Good to get definitive diagnosis. Dont panic, Coeliac Society are great with advice and food lists. Most fresh foods eg vegetables potatoes fish is gluten free naturally its the sauces, gravy etc. Also sneaky gluten can be in some medications and toothpaste! Hope your Son is doing 👍 ok. Happy Christmas, if in doubt just ask members here.