21 years old is a "young adult" IMO, and he is not your therapist. I agree that the fact that he's the only one you have to talk to is not a good reason. I do think you should discuss telling him with your W first.

I also agree with KD that having your son appeal and reason with your W on your behalf is just prolonging the inevitable. If she stays for that reason how does that fix anything?

Accuray

Married 18, Together 20, Now Divorced
M: 48, W: 50, D: 18, S: 16, D: 12
Bomb Dropped (EA, D): 7/13/11
Start Reconcile: 8/15/11
Bomb Dropped (EA, D): 5/1/2014 (Divorced)
In a New Relationship: 3/2015