I think that it's also very important to tell you son your concerns and let him tell you what they do and talk about. Of course keep the conversation on a 9 yr. old level....but he's old enough to talk to about drugs and bad influences and things like that. I wasn't into a lot of bad things when I was young, but my mom and my neighbor's mom smoked and we introduced ourselves to cigarettes at that age (10) so, always be aware of that. It started a habit that I picked up again and lasted for years....
Anyway, communication with him is key. Just be careful of how you approach him. He's probably working against you for his freedom. I know I knew just what my mom wanted to hear not long after elementary school. It sounds like these boys are in high school, probably dating girls, definately going through, or already gone through, puberty....I don't want to be negative at all, but just aware. They also could be respectable kids, but you never know unless you ask your son(or invite them over like someone else mentioned--good advice
). He could look up to them more than he does you just because that's how kids are at that age.
Just follow your gut. If it feels uncomfortable but you don't wanna make him feel like the little kid who can't do anything then just keep him too busy to play most of the time. Give him chores and then reward him with picking something out at the store (once a week, twice a mo...whatever you feel appropriate) money, bowling, sports, excercise with you(like a walk somewhere interesting).....you get it! And then talk to him a lot openly about his interests and throw in where you can the choices you'd be proud of him for making and show him that you are proud of the good choices he makes. It'll make him think when he has choices to make that he likes it when you are proud and he'll know exactly what to do to make you and himself proud. When he goes in the wrong direction, that's ok as long as he feels he can tell you if he's uncomfortable. I felt the openness to tell my mom I tried smoking and she got very upset and told me how she couldn't trust me and followed me around the house....very un-called for. But she should've seen that I was being honest with her, so she COULD trust me!! Her reaction really caused me to shut down and learn to lie better! Now I'm a 27 yr. old mother of 2 and I'm going to try to do things differently, of course....who doesn't! But I want to be more aware of things, including the way my behavior affects them! And I want to realize when I do react inappropiately I can always tell them that I realize it and appologize to show them that I make mistakes and bad choices too and it's ok.
I sure do ramble, I hope someone benefits from my point-of-view and opinions.....take care!