A few ideas:
Cross contamination somewhere
edited to add more:
He could have issues with rice, quinoa or even gluten free oats.
The best bet at the moment would be to record everything he eats in a food journal to see if there is a connection.
Thanks for your reply, soy and corn were the next suspects on my list. It's good to know I am on the right track. During the week that the tummy aches flared up, my husband did the shopping and got soy milk instead of rice milk. He is back on rice milk, but the tummy aches are still present.
Cross contamination is not likely at home, but I do wonder about kindergarten. He has become a finger nail biter, and I wonder if he might be picking up gluten somehow from things that other kids touch after they have eaten. We have holidays coming soon,with six weeks off pre-school and if the symptoms disappear, that might answer the question.
He doesn't have any oats, as they are not recommended for coeliacs where we live. I have wondered about other grains - but hope it's not that. It will really put the squeeze on if he can't have rice. I will get onto systematically keeping a food log.