Really if he pulls a 670 in dealership he probably could get loan pretty easy... although income could be issue....
I would start with Capital One.. put him as applicant 1 and you as 2.
Then take that approval (hopefully) to dealership and find the car you want and let them try to beat the rate you are being offered with Capital One.
If you have a CarMax around you even better... they get approvals in very low scores and it's instant!!!
the big dealerships if you have bad credit and you don't come to the table KNOWING you can get approved you are very likely to see a long hard process that will leave you feeling robbed even leaving in your new car!
or just find a car thats around $2000, put your $2000 down, and drive home.
Financing at sub 600 is pretty much 30-33% down in a lot of cases, sometimes as low as 20% depending on lender (CU's for example might relax the DP requirements). As a result you're shopping in the 6-10K range tops... though that buys plenty of used car.
Auto loans are pretty easy to get approved for if you have enough down payment, the rate might not be stellar but you can refinance later. In your case I'd suggest getting the auto loan rather than just buying a 2K beater.
Try Ally Financial. you can walk in dealership with preapproval. If terms of preapproval are not to your liking, the dealership can convince Ally to change them. They did it for me.