Add my name to the chorus of "He needs to know the unvarnished truth." This is not always the case in these situation, IMO, and sometimes discretion is called for, but here, he pretty much knows or strongly suspects the worst of it. You don't want him to end up resenting you, too, because you kept things from him or failed to take steps to protect him from any kind of taunting or bullying at school because of the situation. The way you go about handling it is delicate, though. Don't necessarily want it to come out as a vindictive you "diming your wife out" and trashing her to him. Still, HE has asked. He needs to know truth, particularly if he asks YOU. You CANT lie to him. Not saying he needs to know all the grim and dirty details (when, where, specific acts, etc), but he needs to know the situation.
ALSO, VERY IMPORTANT-- since he knows and will likely know more, you need to show him what a strong, confident man and father looks like, here. Don't be bullied by your wife, and don't take any of her crap. Doesnt mean you have to be a jerk, just... strong. This is an opportunity, even as your son is suffering, for you to provide a great example for him that will help him down the line in his own life and relationships, and could very well protect your relationship with him from succumbing to a rift that might be hard to bridge down the road. Best to address the situation NOW, and do so decisively and confidently. Not sure i know the exact words to use, here, although i think Sandi2 has addressed it from time to time, and there is some discussion of it on my threads which i don't have time to go dig out right now. My sitch was somewhat similar to yours in that sports teams were involved-- my sons played HS football with OM's son, and both of us were part-time volunteer coaches for team. So I myself had to contemplate this prospect myself, in some depth, but, thankfully, it never came to me having to tell anything to my sons as the matter never became public and my wife broke off the affair before it became as involved as the one your W is in. And, obviously, i ended up reconciling so "all is good" I am pretty sure that one of my boys suspected what was up, but W ended affair and we turned it around before boys had a chance to find out. I confronted OM and called him out on it and warned him off, in semi-public but not publicly, and told my boys at some point later, after OM had spoken to one of them, that he was "no longer a friend of this family" and when asked why: "Details don't concern you" (And at that point, they didnt) "Sometimes people you think are your friends aren't really your friends... trust is important to me and he turned out to be someone i couldn't trust. In that regard, I felt i had protected myself and my relationship with the boys, and set a good example if somehow later it did come out. As to the prospect of telling them about the actual affair, i was repeatedly cautioned by others not to do so "as a weapon" or for the sole purpose of "getting back at" my wife, which i thought was good advice, but, at the same time, received alot of advice not to lie to them. It never came about so can't say for certain what i would have done and said, but i did tell W on more than one occasion: "I am not going to lie to our sons", and i was serious about that.
Good luck, God bless... This is a bad situation with no painless path forward-- sending up prayers that it turns out for the best for you.
H53/W51, R-ing 4/'18
"Do not arouse or awaken love until it so desires"-Sg.of Sg 2:7
"So oftentimes it happens,that we live our lives in chains, & we never even know we have the key"-Eagles III 1:3