Could be sulphites or preservatives in food that do it.

DS has a sore tummy often and it's mainly associated with his growth spurts so the Dr thinks it's possibly hormonal. I find a homeopathic remedy called camomilla useful. It comes in tiny vials and he has a few a balls of his 'placebo' and usually feels better. One day he's going to figure out what placebo means lol.
Persistent sore tummies are very common in children and often they can't find any cause but they vanish as they get older.