I think it all depends on your child. Dose he folow rules, have good road awareness and stranger danger? I personaly think the longer you dont let your child out on their own it causes more problems for them when they are older with regards to confidance when away from their parents. I have a friend who is 20 and still lives at home. She lives a 15 min walk away from me and the walk is along a main road and she is not allowed to walk back from mine after half 9! shes 20 ffs! As a result she wont go on holiday with friends because shes not confidant enough to be without her parents. She wont go out clubbing futher than the next town as she is to scared.
Ok i know the examples iv gave are a bit on the extream point but i think you get the idea im trying to put across, as much as i know you are trying to protect your son sometimes protecting them to much can do more harm than good. Why not let him go out for half an hour before tea and the day he comes home late hes not alowed back out and then after a while up it to 45 mins and so on.