If you do have sushi, avoid soy sauce - it has gluten. Though you're avoiding rice anyway, so you probably wont.

Have you looked at the GAPS diet? There are some recipes in the Internal Bliss recipe book that can do as snacks. It's extra work, but things like making your own chips from sweet potato or beetroot could work.

Can he eat almond meal? There are often GF muffins available at cafes etc. Or can you pack small GF crackers with some cream cheese to take with you?