I'm not really "back" per se, but I did want to check in on you because you were so generous with me. Maybe I can offer you some help here because I have an 11 year old son and can understand the bind you're in, as I also strove to keep W's wanderings secret and to try to shield the kids 100% (D13, S11, D7).
We often read that "honesty is the best policy", but that usually refers to interactions between adults. On the other side of that is the need to "protect our children" by sheltering them from truths that they may not be ready for, or that they may not need to be aware of because it hurts them more than it helps them. Having a 13 year old and an 11 year old, there is a decided loss of innocence over the course of those 2 years, and at 10 you're definitely dealing with more of a child than a young adult, so there is simply more to protect, and probably less virtue in brutal honesty.
That's a long way of saying "don't beat yourself up over the lying" -- yes, you have misrepresented the situation to S10, but you did it in the interest of protecting his innocence and his faith in his family, not to try to deceive him for your own gain, which is the motivation typically associated with "lying".
The issue you now have is there are so many people involved, and the situation has been going on for so long, that trying to keep S10 protected is starting to become impossible -- it's like a stress ball where if you push it in on one side, it just pops out on the other, so yes, the time may be here to share the situation with S10.
In that context, you're struggling with how to represent it to him, because he'll undoubtedly have questions. My sister got divorced last summer, and they went to see a family therapist for advice on how to discuss it with the kids -- you probably would be well served by getting that kind of professional advice. I think it's clear that you don't want to paint either of you as the instigator or the problem, but rather to portray it as a challenge in your marriage that you both have a role in, that you both continue to love each other and S, but that at this point you don't know how things are going to turn out, and that uncertainty is okay. I'm just winging it here, a pro could do better.
Don't beat yourself up about "lying", look at it as "protecting" that was done with the best of intentions. You shouldn't try to "protect" your spouse by lying to him, but you're dealing with a child here, and in that context it's perfectly appropriate.
Good luck zig, challenging times.
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