Hi M.,
I can relate to your problem. I am amazed at how comfortable other parents seem to be about letting their children play for many hours without supervision and only vaguely knowing where they "should be".
When I was your child's age, I walked and biked all over the town. I am sad about it, but I just don't feel it's safe to allow my kids that freedom. Some people think I am overprotective but I know where my kids are and know that they are safe .As a parent, I believe that is my job. I will teach them to deal with the real world in little steps as they are ready. I am willing to put out the effort, (and it is a lot of effort), to provide the extra supervision even though it would be a LOT easier to be like so many other parents whose kids are out playing and they can get on with their business.
My son is just 9. He has a friend who, when at their house, is allowed to play outside on his own and walk to the park and creek near their house. As long as they tell their parents where they are going, they are allowed to be gone for hours. I have told my children that they are not allowed to do that. I had previously told the parents that I am just not ready to let my kids go unsupervised and made myself out to be the one with a hang up even though I believe I'm just being sensible. One day, I went to their house to pick up my son and his friend's mom was sitting outside her house, likely supervising my son because they wanted to be outside and she knows my rules. The boys were laying face down on skateboards and, from 2 (short) blocks away, pushing themselves off and riding down the street back to the friend's house. I was horrified! They actually had to cross a road doing that. It is a fairly quiet neighborhood but a car could easily run them right over by not even seeing them fly by. They were being "supervised" but doing something so dangerous! I have explained to my son why I will not allow him to do that ever again. My kids don't get invited there as much because the kids have to be trapped in their house now when my kids are there and no one likes that. I have friends over here and everyone is happy. We bought a house that is small but has a huge backyard especially for this reason. My kids can have all sorts of adventures out there and I can know they are safe. They climb trees, make forts, play with the dog, create inventions, hide and seek, and have all sorts of other fun but they are not going to be hit by cars or approached by strangers.
I have been feeling guilty that my kids have not had some of the adventures I had as a kid. I used to play for hours in a creek in my neighborhood alone. My son wanted to go to the creek near our house so I took a friend of his and him there to explore. I brought a book and sat off alone under a tree to let them play without mommy being in their way. They found a rope swing and my son, showing off, immediately flew across the creek on the rope swing and it was too fast and he wasn't watching where he was going so he slammed into the creek wall when the rope returned to where he had started from. He had a very strange look on his face and made some unusual sounds then staggered into the creek and fell into the water face down. I thought he may be playing but he didn't get up so I ran over to him and pulled him out. He was unconscious and pulling him out made him start to come to. His face was covered in mud and he stared at me blankly. He had no idea what had just happened. In a few minutes, he was fine and back on the swing being much more careful. I sat there stunned knowing that if I hadn't been there to pull him out, he would have drowned. His friend is a sweet, gentle boy who had just stood by as he didn't know what to do. He may have tried to help my son had I not been there, but not sure if he could lift him and if he would have reacted in time. My son is pretty mature and responsible most times, but he's still a foolish kid and needs more guidance before he can be let off the reigns entirely.
Sorry to have gone on but my point is, tell the parents that you are just a worrier and you don't let your son play unsupervised yet. See how the parents react and if they don't "get it", then just invite the friend to your house all the time. Don't be afraid to offend or be embarrassed. I know you'd rather that than a call from them saying he's in the hospital after being hit by a car or worse. Good luck and stand strong!