J.A.
My nephew is deathly allergic to peas, peanuts, and soy; however- he can handle foods "made in plant that processes" those things. He is now in 7th grade, but when he was little, his parents had to make most of his food from scratch. As he got older, they let him try things that had a little bit of soy in them, like soy sauce, and he has been fine. Pure products still make him feel sick to even smell them, but he could likey have a little and not be harmed at this point. (I have no intrest in testing that theory!)
Chances are that as he gets older, he too will be able to eat your general legumes like green beans and pintos and such, but if it is a severe allergy (the kind they give you an epi-pen for)then peas, peanuts, and soy are forever off the menu.