Welcome to Psych Central, lane. And thanks for sharing your concern here on PC. At least on the surface I certainly don't see anything wrong with dropping your son off to hang out with his friends. That, of course, assumes that the hanging out he's doing doesn't involve alcohol or drugs. From my perspective, I see no value in punishing your son for what happened.

There are a couple of (rhetorical) questions I would have though. You mentioned the police thought your son's accident may have been intentional. Of course, that's just their professional opinion. Assuming your son hasn't said it was intentional there's no way to know for certain. I would wonder, though, what kinds of services e.g. counseling or therapy services, your son is receiving. Even if the accident wasn't intentional, it was still a traumatic event. And if by chance it was possibly intentional then that makes it all the more imperative that your son receive some sort of mental health services (assuming... he is willing of course. You obviously can't force him.) Depending on the severity of the brain damage you mentioned, this may all be of particular concern I would presume.

There is a potential danger here, it seems to me, that facilitating your son's hanging out with his friends, while he presumes the accident was just an unfortunate occurrence that is now behind him & is of no further concern, may be setting him up for further difficulty as time passes. As I wrote above I certainly see no useful purpose being served by punishing your son. But neither, I think, do you want to end up enabling him to perhaps set himself up for further tribulations in the future. My instinct here (whether rightly or wrongly) suggests to me, there may be a fine line you're going to have to tread.

Here's a link to an article that may be of interest, from Psych Central's archives, on the subject of traumatic brain injury:

Traumatic Brain Injury: The Hidden Epidemic Nobody Wants to Talk About

I hope you find PC to be of benefit.