Psychomum has a probable coeliac son - but she is away this week! I will save this to my watchlist though (or maybe even log in and save it to hers) so that when she does get back, she will be able to talk to you (and hopefully the other mums that reply).
Oh, Jacob is allergic to milk too, but he does have soya milk. It is hard, but from watching her, it seems to be something that you just adapt to and get used to.
Oh, and as much as everyone on here hates MacDonalds - it might help you to know that they know exactly what is in their foods, can give you a list so that you know what he can have, and are very helpful with cooking it etc - even if you do end up with just a plain burger, no bun, and some chips.
Pizza hut is the other good one - if you take your own gluten/wheat free base, they will topping it and cook it for you.
I know you probably don't do either on a regular basis, but it is handy to know .
Jacob is very low weight too - he has always been diddy like yours, but is developing fine, just small iyswim.