Be careful what you wish for Allison64. A lot of council estates are not what they were in the 1970s. The sort of children who are playing out on your parents' estate in 2010 are more than likely probably not the sort of children you would want your DS to mix with, even if he had the chance to.
My mother has council houses near to her and when the elderly tenants she knows who have lived in the houses for years move off into sheltered accommodation, or die, they are replaced with what can only be described as the lowest of the low and their children can only be described as feral. No one decent ever seems to come to live there anymore.
I totally agree with you that not many children are "let out to play" as they would have been 30 or 40 years ago, and that is sad. Don't stress about this because your DS certainly won't be alone in his situation and I'm sure the vast majority of his classmates wil be in similar position with having parents who are not comfortable with letting them play out alone. Times have changed and it's just a sign of the times I'm afraid.
My DS has a lot of friends who are in "sibling groups" as you call them and they still complain of being bored and find they spend a lot of time on their own too. Some have big sisters or brothers who they have nothing in common with, or who find them annoying, whilst others have little brothers and sisters who they find boring and annoying. Children get to an age when they're not always that happy to play with their siblings like they did when they were little. My own neice and nephew who are very close in age are constantly squabbling and fighting and don't seem to get on very well together at all for the most part.
So the grass is not always as green on the other side of the fence as you might like to think it is...
My advice would be let your DS be for now. In time, he'll be old enough and independent enough to meet up with his own friends and do his own thing. He'll find his own interests in his own time. 7 is still quite young to have developed firm interests. Trying to push him into things and getting constantly knocked back by him will only add to your frustration. Just leave him be to do what makes him happy for now and let him come to you if he has any ideas about what he'd like to do.
Everything will work out, you'll see, and one day you'll find yourself looking back and wondering what you ever worried about.