He opens his mouth and, moving to precisely time it, swallows the grenade. Perhaps the massive grinning ability helps with that rather odd feat. Then a grin, as the concussive force is apparently dulled by the black hole inside him.
Bits of shrapnel do pierce his skin, but there's no serious damage. Maybe aiming for the face wasn't such a good idea.
"I'm sorry, what were you saying? Mouth was full."