My own feeling is that if you've trained your child well and he's sensible, and you've told him not to open the front door to anybody whilst he's on his own, then he should be OK. Children are so protected these days that if they don't have the chance to be a little independent, by small degrees, in a safe situation, they will never be confident to deal with life. But if you're a little anxious, can you arrange with a next door neighbour, or neighbour opposite that if your son has any problems, he can telephone them and they will come to his aid. As long as he has the phone numbers handy, I think that's a reasonable compromise.