Check the unarmed combat section. I think any unarmed damage it's just Brawn + successes, but I don't have time to confirm it currently.

Correct, he is an NPC.

If he's an NPC, just arbitrarily set his base damage to what suits the story. Your players don't see the NPCs stats so they don't have to be justified. Don't make it so high that he'll drop a healthy PC in one shot, but enough to make them scared of getting close again.