He may be craving it because it’s fat. If he wasn’t getting enough fat, it’s a pretty easy source of fat for the body to crave.
Doesn’t sound like it’s an allergy, exactly, but it could be an intolerance. There is a lot of aflatoxin in peanuts, so that could be the problem. Maybe try almond butter instead and see how he does?
Some would say it’s due to the high amount of PUFAs, but I am skeptical that would be such an issue on one serving a day.