I have seen and heard all of these variations. I would say they're all correct.

Hello Tyker! According to the Collins Dictionary it should be written like this: "six-pack" and this term has two meanings so the sentence should show the context. So the correct way of writing it is: "He has a six-pack stomach." or "He has six-pack abs." and "They have six-pack abs." I hope that helps!

All doog. BTW: He has a six-pack, without context probably means he's carrying beer.