It is always hard to find a balance. Pot can be serious problem for some kids, it is also true that a lot of kids smoke pot as a social thing and it does not become a problem for them. Certainly the fact that your son was suspended is a red flag but given that he is basically a good kid, a good student and an athlete he has alot going for him.
So this is my unsolicited advice.... focus on his behaviors not on the fact that he is or is not smoking pot. I think focusing on the pot smoking (given that he is 18) will only lead to a difficult relationship between you. You want to do the best you can to keep a good relationship with him, but at the same time not condoning his drug use in any way. It is a hard balance to be sure. I dont think you want to have to act like the police on this issue.... certainly though if he using your car you may want to limit that as it is pretty darned dangerous to smoke pot and drive.
So I would keep watching how he is managing the rest of his life and go from there... if his grades start dropping, if he loses all motivation, if he stops taking care of his personal hygeine, if he starts hanging out with different and less savory friends then pay attention and address those issues. All of those things are likely to happen if he is getting into serious pot use, and less likely if he is just smoking it once in a while as a social thing.
TL