Cherie, I think I might be in a situation more like yours than some of these other mamas. I've seen a very strong sense of trust and responsibility and respect develop in my kids because I don't "lower the boom" on them for no reason other than I want to.
But I can also understand your wanting to see your son!
If I was in your position, I'd invite the friends to come over to my house for a while... just let them know you're feeling lonely and left-out, and give it maybe a day so DS can help clean up and plan some food and so on.
Talk to the other mom, let her know her kids (and HER!) are welcome to come visit at your house, that you'd like to get to know them better, and you want a turn hanging out with your kid, that you need him to help out with the house a little bit and you just want to hear what's going on with him.
My DS just started his first relationship with a girl, they're both 13. I don't think I'm ready for him to be spending the night at her house, but both kids have already spent a lot of time hanging out at each other's houses. And DS does spend the night at his guy-friend's houses... if it was a sister of someone he'd already spent the night with, well, if the mom was on board with nothing too out-of-hand going on, then I'd be OK with him spending the night there.
As far as what is "out of hand," our definition is that you don't do anything with your girl/boyfriend that you wouldn't do if I was sitting right there in the room with you. I know kids need to explore their sexuality, but I'm afraid I'm still enough of a prude that they're gonna have to grow up enough to get their own place (or pay for their own hotel room) to do anything more than snuggling on the couch. If you're old enough to have your own place, then you're old enough to deal with the consequences of sex. If you're not old enough to have a regular place to go to for nookie, then maybe you're not old enough to deal with the fallout!
You might want to check with the other mom and make sure her values are near yours in this department.