How old are the man's baddies? Weigh the cost vs what you can afford and how much longer it will be on the credit report before negotiating. In the meantime, does he have any positive tradelines?
Also, add him as an authorized user on your good credit cards (old cards and especially AMEX if you have one). Make sure he understands the huge favor you are doing for him...but don't give him any of the cards to use or he will ruin your credit too 
If you really want a car loan, go apply somewhere else as your single self
His credit does not affect you if you don't tell anybody about it.
Maybe not the "best" advice but it works if you really need a car.
...the negatives will remain on his CRA records for 7 years from the last date he paid on the accounts ...that date will be shown in the CRA reports ...it is unlikely that a CA will remove negatives for $8k, they paid hard money for his debt accounts and want something back.
...that said, it behooves you ...and him ...to begin to work to repair his credit ...there is plenty of good advice here in the forums already on that process but it takes time, patience, and persistence ...work on each negative one at a time, step-by-step ...there is no magic wand but there is a rainbow at the end
...my immediate concern would be with your CU ...you apparently have a long relationship with them in good standing ...I would be asking your LO for specific advice on exactly what you need to do to get the auto loan you want now from them ...a good CU LO will help you, if not, ask to speak with another or even their superior ...your situation is specifically why CUs exist ...don't settle for less.
...as others have noted you can always find an auto loan from a dealer if you can document income but the rates will be abusive compared to the normal CU rates ...hth
