Are you sure this is why they're letting him go? Check your facts and then if your son can, have him sue for discrimination.
Not saying your son is a bad guy anymore, but unless you go to work with him and sit there with him, you don't know how good his work really is, other than what he tells you.
BK has rules and regs. My question is, did he franchise the BK he was working in? He was the owner? How can they let the owner go? They have to buy him out. Your story is a little "fish sandwichy".