In order to qualify for a refinance, your son must have steady income and a good enough credit score to get a loan on his own. If this isn’t realistic, you could refinance the vehicle and switch your role to that of a cosigner or guarantor, which means you will only have to make payments if your son absolutely can’t.
If he doesn’t qualify for the refinance, work with your son to improve his credit score over time. Hopefully in a year or so, your son will have built enough credit history and have the income to take over the loan fully.