We can speak to this one as we've been here before and done it (and still do). If he is of legal age (which he is) it is 100% perfectly fine to fuck him. You have to understand that it consensual among both parties and there is absolutely nothing wrong with it. However, there are significant challenges and risks you have to be willing to take which might make you think twice about it. Some of the issues with this:
1) Your son could find out. This is the most obvious concern. Do your best to mitigate it by not allowing them to take pics or vids which is not easy with a lot of the kids today. If they decide to tell your son or his friends and it gets back to him - deny, deny, deny.
2) The dynamic is different. It's quite standard for some of the guys to try to dominate the wife and humiliate the husband in the cuckold lifestyle. That is still somewhat the case here but there is now more of a shift to these kids thinking they are humiliating our son. They'll often bring him up during sex or make her say she loves him while they are facefucking her. It's not so much about dominating another man's wife as it is about dominating your friends mom.
3) It'll likely be much rougher. Younger guys are immature and watch a lot of porn. I think because of this the sex is rougher in general. But add the cuckold scenario and the fact that it's their friend's mom and that only adds to the intensity. My wife has had plenty of rough sex but the roughest by far was from a couple of our sons friends.
4) You're leaving yourself open to blackmail. They're young and dumb and all they care about is fucking. They also like to take pics, vids, and brag and they might use this against you to get what they want. My wife was in a MFM with 2 of them and they were teasing her that they would video call our son if she didn't do certain things. They ended up not video calling him but they did call him during it and thankfully he didn't pick up but the left a voicemail of them fucking her. My understanding is that it was just gagging and sex noises with no identities revealed but it still goes back to dominating and humiliating the mother and son.
I am not trying to discourage you. Not at all. It can be a great experience but it carries real risk. And the more that get involved the higher the risk. But if you have someone who you feel is more mature than their age then that is probably a good choice.