To him: Develop a budget and a system to handle bills, etc so everything gets paid on time. This can take some trial and error, that's ok. It's a learning curve.
To you: Make sure he knows when he needs to return the car to you - give him plenty of notice so he's not blindsided. If there's any money or goods you're willing to give him, let him know specifically what that looks like, and any obligations on his part. If he's living with you, be open, transparent and discuss about your expectations of rent, cleaning/cooking, etc. If you want him to move out, give him plenty of notice of that end date and let him know what, if anything, you'll do to help (if requested).
Answer questions if asked. Offer advice if requested. Tell him you're more than happy to help him out, but he needs to ask you, you're not going to intrude. Mostly, let him figure it out. Pretend that he's another adult, not your child, and treat him accordingly.