Buy him a mobile, a cheap nasty basic one, and tell him he can go out with his friends if they call for him, so he'll ring them, learning how to use and answer a phone, and being a cheap phone won't be a target for older kids.
The view is not great, try and get him into some after school stuff, can never have enough freinds to mess around with, my lad was doing ju-jitsu by age 4 or 5, met a lot of freinds etc in after school things.
The best thing is to get to know as many people around you as you can, others being parents will all muck in and watch each others.
You are right to be concerned, it is a very worrying time when you first let them "loose" as it were, but does them no good being kept in, maybe if you have some freinds on the lower floor ask them to also keep an eye out etc.
tabetha