Why does it hurt the tip of my boyfriend's when I jab him in the bellyon?

My boyfriend and I always play rough, wrestling and such, and he does this thing he likes to call "bobos" where he jabs his fingers into the crease of my abdomen and thigh, and it hurts like the ens... anyways, I always retaliate by jamming my finger into his bellyon, which always ends the game when he shrieks "OW, MY !"... as vulgar as that sounds, I really don't understand the connection. I mean, I don't disbelieve him, but he says that, when his bellyon gets poked, it "feels like lightning hits the tip of [his] " and it is apparently painful enough to debilitate him completely. What would this be caused by?