If you’re a co-signer, the loan is also in his name and will improve his credit score over time. The most desirable features of a credit history are length, size, and quality. The rules for getting there are “borrow little, borrow often, and always, always pay on time.”
In my opinion, the best course is to get a new credit card as soon as possible, and use it. This will almost certainly be a high-interest card with an annual fee and a low credit limit, but the interest doesn’t matter if the payments are on time, and the limit will go up if you ask nicely, and sometimes even if you don’t ask at all. Years of good behavior will create a more reassuring history.
Credit utilization is one of the biggest factors in determining credit score, and ideally it should be about 10% (that is, of the credit available to him, about 10% should actually be used; for instance, if he gets a card with a $300 limit, he should use it to maybe buy one tank of gas a month). Using all of the credit available to you is a red flag, but not using any is at least a yellow one.
The auto loan will initially hit the score hard - his utilization will be close to 100% - but as the loan is paid off, this will decline, and offer greater opportunity to hit the mark in both utilization and the other huge factor, payment history.
Another thing that raises the credit score is having a good credit history before the bankruptcy, as that actually determines most of the credit score, but that’s too late now. If he hasn’t already closed existing lines of credit, that’s a big DON’T. Anything he has managed to maintain from before the bankruptcy - like a credit card with no balance - will add to the length and quality of the credit history.
This, of course, presupposes that he actually has good credit habits and fell afoul of something too big to absorb. If he’s actually not financially responsible, that auto loan will not help him much unless you watch it like a hawk, and it will hit your credit score too.