The "knife" theory could be partially true. It is a sensitive area, and the taint piercing was a recent, and perhaps not 100% healed injury. Suppose he thinks he can grow a vagina, so he pokes and prods at it. It is right in the undercarriage, so even looking at it could involve some serious prodding if he wasn't super careful. This makes the wound appear, confirming his vagina beliefs. So he pokes and prods some more, maybe with some sort of instrument, because he is excited about it. That makes the wound grow, and he gets more excited, creating a vicious circle.
The self-harm theory doesn't need to involve him taking a knife to his crotch and carving away. The combination of his voodoo beliefs, the general sensitivity of the area, a pre-existing wound, questionable hygiene, and general clumsiness and cluelessness could lead him to accidentally self harm.
It is probably less dangerous, because he is less likely to accidentally maim or kill himself with one mistimed slice, but it is also probably harder to get him to stop.