I would say (and im a parent myself) that the kiddie would be safe to play out side with his mates to a reasonable time, maybe 6.30ish WHEN he understands the problems that come with society,obviousely not in every detail.

ie: does he undersatnd fully not to go with strangers/or off alone?
Does he understand the dangers of roads/strangers/dogs/other kids who might bully?

If you can make him street wise (a little and i understand hes very young) then you might be able to relax a little.

Get him a cheap mobile phone maybe, something from asda for £10 that you can ring him on or more importantly he can ring you, this alone can install confidence in both parents and kids.

But at the end of the day, he is young and maybe a few years older would be better and it does depend on the area you live in.

You can become over protective as a parent and i must sound quite the hypocrite when i say, you must protect them for as long as you can, but then again you cant smother them,again i do understand hes only 6 yrs old and it must be difficult to even contemplait letting him out alone.

At the end of the day, its down to you and your missus and NOT what people think on here.

Good luck on your decisions.