I would avoid the therapist, but I am not hating on LH or Neffer here. I'm leery of therapists.
But I agree with the others, that it has reached the point where you just tell him. Don't badmouth your W like she did you. Kids pick up on this. My dad badmouthed my mom several times during and after their divorce, my mom never did. Now her hands weren't perfectly clean but she did this part right.
99% of the sitches we see have an OM/OW. I'm sure my parents' divorce involved one. So I'd say to your boy that it's nothing out of the ordinary as far as divorce goes, but it hurts like hell. Give him a big ole hug and tell him how y'all are getting through this as a family (that doesn't seem to include your W currently) and are going to focus on doing the right thing every day.
H 34 W 29 BD 3/12/18 Divorce Busted Spring 19
It is not things that bother us, but the stories we tell ourselves about things.