It's totally natural to worry about your son. As others have suggested, set boundaries. Your son might be an excellent driver with you...but he is STILL a teenager. Teenagers are generally not known for their superb decision making skills.
Frankly, both my husband and I find it hard to believe that, as a society, we let 16 year olds drive cars. Our daughters will not be driving at 16. My husband is from Europe and you can't even get a license until you're 18. Anyway, we understand that each family has to make their own decisions.
But, driving is a privileged and HUGE responsibility. You are not only responsible for yourself and anyone in the car with you, but also for others on the road. Driving is one of those things where YOUR mistake CAN affect other people in a very serious way. So...boundaries boundaries boundaries. And consequences. In my opinion, driving for teenagers is one of those things where there are no second chances. You mess up once you loose your driving rights for awhile. That's it.
Also, I don't know what the laws are here in Ohio, but we just recently moved down from Michigan. I know at 16 you CAN'T drive with friends who are under 18. If you have someone in the car, then someone HAS to be 18 years old. Otherwise, you drive alone. So, make sure you know what the state laws are pertaining to having passengers and being a passenger. I would also make it a rule that there is NO cell phone talking while driving.
Set the rules. Sure, they may be hard to enforce, and much of it will be on a trust thing...but that's the rub. If you DO find he has broken the trust...then he has not only broken the rules...but lost your trust. That's a big deal and should have big consequences.
Anyway...that's my 2 cents.