Looked at the post by Raven and that was my daughter too, now I look her and this is my son.
I get the feeling that he likes to be the boss, my boy does and this could be a way of controlling you, I only think this because when your not around or the p/b jar isn't he is more than happy to eat other stuff.
Or it could be his teeth my boy eats alot of p/b because he has a bad tooth and the p/b sandwich doesn't hurt it but that is being fixed shortly.
In the end he will get tired of it and go to other stuff. I'm not ashamed to admit when I was a Kid we had lots of chooks and I ate omlets every day for 2years I still love them but I'm over the obsession.