Okay, so chicken is probably safe. Welcome to the life of us with food allergies!

Try feeding him just chicken - boiled is fine, nothing fancy is necessary - for a few days, with some brown rice for some bulk. See if that eliminates the gassiness. Then read the ingredients on the bag, and see what else is in the food, and see if you can add one ingredient at a time to the chicken and rice (as long as that leaves him gas free), and you should be able to work out what the problem ingredient was, and then look for a food with just things you know are safe! If chicken and rice still makes him gassy, try chicken and potato ... you get the idea!