Hi M.: I know how you feel, we my daughter turned 16 in December, but we didn't have her get her license until April. We felt she still needed more practice.
Feeling nervous is normal, we're all driving around a 2,000-lb. weapons! The only suggestions I have are start out driving with him on shorter trips--10-15 minute jaunts. Maybe you could drive somewhere close and focus on having him practice parallel and angle parking. The better he does on these shorter trips, the more you'll probably calm down.
Try to control your voice, force yourself to keep a calm tone when you give suggestions like "Keep a good distance from that car ahead of you, they seem to be hitting their brakes a lot," instead of short burts in a panicked voice, "Watch out for that car!!!" The calmer you sound, the calmer he'll feel.
Also, I don't know your religious background, but doing some deep breathing and lots of prayers helped too!