Check the Capital One Auto Lending site and see if your home state is one of their "pre-approval" states... for some states, they are offering soft-pull preapprovals (the same kind they will do in any state on a new car loan). He can enter his information, including payoff amount, and they will give him an interest rate quote, with no impact on his credit. This would tell him if he is in a better position, credit-wise, or not yet, without incurring a pull on his record. If it's a good offer, take it - I can tell you from recent experieince that is is a very painless and almost paperless process - and if not, then just don't accept it and try again later or try your credit union, or whatever. He's already doing business with one of the sharkiest of the sharks in the industry, so he certainly can't do worse...
I would personally not cosign, regardless of the stability or commitment of an interpersonal relationship, just because sometimes life happens - to one or the other of you, if not both at once - and you may need to not be tied to this loan if you guys suddenly need a new lease/mortgage, or you yoursel fneed a new loan, or there is a career shift, or whatever.
