kids deal with things so much better then we do. I don’t think he’ll get so  stresses that he’ll be effected by this I think it is great that you are going to tell him, I also have two children who there father chooses not be involved with I am also married, My kids do know there bio father but they call my husband dad for my daugter thats all she’s ever known. But when we have these wonderful men in our lifes who love our kids like there own the kids should know this, it’s much differnt when they love our children not because they have to but because they do kids like ours are very lucky to have daddy’s who love them so much Good luck