I think that if he feels his dad spoils him rotten, let him live that fantasy for right now. It hurts you -- I know -- but soon enough, he will realize that presents twice a year isn't spoiling someone rotten. It's a tough journey, to get from "why didn't Dad want me?" to "Dad is a sick man and it's got nothing to do with me" -- and he will probably take it, eventually.
But for right now, just let him enjoy the little attention his dad pays to him. Even though it hurts.