The 3rd one just doesn't sound right. It still sounds like he's still at the university, but no longer playing football.
Saying he's an Ex-Sooner means he used to be on the team, but isn't anymore, IMO. Saying he's an ex-QB, to me, means he quit the team but is still taking classes (or something to that effect).
I would assume he hadn't been in school for a while, so saying he's an Ex-Sooner makes more sense here, as he no longer plays for the team. You can still be an alumni but be an Ex-(enter sports nickname here).