If he's mentioned the stuff about Warhammer 40k, then I suggest you tell him his next opponent is the result of a Gene-stealer after meeting a dragon's god.
Genestealers are basically the aliens from the aliens movies. Just take whatever it is they've eaten and make it ugly, feral, insanely more powerful and faster, and give it a tounge with a mouth on it. And the genestealer he gets to face had dragon's god for a snack.
If that doesn't work, Rescue Him!
He some how ended up in your game f'ing things up, Space marines don't leave thier men behind! A space ship swoops in, extracts him ( forcibally if neccesary, with equal powered troops, space marines are also made in batches of too d**n many ), and whisks him away from you savages and back to the saftey of the future. To the theme of a Hughey Lewis song, if you like.
Gonna go back in time! Gonna go back in time! Yha!