he has a diet set out by doctors, he follows it very carefully also. is there any supplement products he can use like protein or weight gainers
In terms of weight gainers, he can have ANYTHING that doesn't have gluten in it (also avoid Malt and Modified Food Starch) eggs/egg whites, egg protein, whey protein (I'd recommend a totally unflavored whey protein, the garbage they put in the flavored ones can conceal possible gluten containing items) milk, meat, brown rice, quinoa, millet, corn, veggies, fruit, nuts, etc., etc. Basically, all whole foods with the exception of gluten containing grains.
Additional considerations if he's not gaining weight:
A) He's not eating enough -- have him measure his food and log his calories, seriously. Sometimes it's really something THAT simple. Some people new to a GF diet find it bland and get bored ... they don't eat as much as they normally would.
B) Maybe there's something else going on, possibly an intolerance of something else (it's very common to be intolerant or even allergic to more than one thing, particularly with celiac).
Finally, if he's only just started following a GF diet, as I said before, it can take up to six months for healing and until his insides have healed, he's might have trouble bulking because his body is currently busy repairing other stuff.
Oh, one thing that I just remembered, in terms of supplements he should be taking, in addition to a good multivitamin, get him to start taking B12 (tell him to get a sublingual spray, not the tablets, they're worse than useless) and vitamin D (1000 mg a day). Anyone who has bowel disease or malabsorption issues should be on both of those supplements as a matter of course (whether they test low for them or not) whether a doctor tells them to or not.