I think you should do it now but not go into too much detail more than is needed. Kind of like the sex talk....tell him what he NEEDS to know right now and no more. I would tell him that you were married to a man who didn't love you like your dh does now. Tell him that you didn't want him growing up in a hate filled house or something to that effect and then go into the positives about his life and how much his "dad" loves him! It's going to be hard! My bro and sil adopted a boy and he is now 11. They leveled with him from the very beginning and thought all was well. Well they adopted him because she had my niece who is 20 and then had several miscarriages and then lost an ovary and a fallopian tube due to endometriosis. They thought they'd never get pregnant and the doctors all told them that the chances were very slim. Well at the now age of 47 they have a biological 2 year old that was a total WHOOPS and my adopted nephew is struggling with it right now! It's never easy but I think you really need to level with him and tell him as much as he should know! I wouldn't go any deeper than needed and I'd let him guide you once you open up the door. I don't have a lot of advice on how to open the door but I think once you do you will be relieved and he may be also!
Good luck to you...I know this will be hard at first but with a lot of love and support from your whole family it can be done! Stay strong and level with him before someone else does
Heidi 
Good luck to you...I know this will be hard at first but with a lot of love and support from your whole family it can be done! Stay strong and level with him before someone else does
Heidi 
