The oat cakes (can't remember their name, something scottish - McNairns?) in white boxes are meant to be really pure with v.little gluten in. It says on the box that they are tolerated in small amounts by most coeliacs.
I tried DS on a quarter of one per day for a few days, and he got ill again, which confirmed my belief in his allergy.
Good luck. It's important to me to keep trying (every three months or so), because I would hate to exclude something from his diet for years which he was actually OK with. It sounds stupid, but because I haven't managed to convince the GP to get tests done, I sometimes feel like I've got that Munchhausen (sp?) thing about his food intolerances.