I am not sure when or how you should tell him, but I wanted to share our family's issue with this. My mom left my dad after 35 years of marriage. My dad was really ill and was in the hospital. While he was sick, I heard through the grapevine (friends of 1 of my brothers) that my oldest brother was adopted by my dad (at what age we don't know). I did confront my dad, and he confirmed it but declared very passionately that my oldest brother is very much HIS son. Since my dad was in the hospital I didn't push for answers at that point. He died a few months later and my mom 8 months after that.
Now my oldest brother has, and has always had some mental issues, on top of ADHD. When I found out the news I didn't, and still don't, look at him ANY different, he is still and will always be my brother, and my dad had raised him since he was 3, so I feel like my dad, is OUR dad.
Through this whole ordeal he will only talk to people he knows for sure knows he was adopted (our aunts). He has expressed that our mom told him to keep it a secret, and me and our other brother should never, ever find out. Now, after 2 years after our parent's have passed away his biological family is contacting him, which is causing him un-needed anger, and anguish. He is still having a hard time dealing with our parent's death, and now this.
I personally do not know why our mother insisted on keeping this from everyone, or why our dad went along with it. All I know is, now that they are gone, my brother is hurting, doesn't understand why some of the family knows this secret, his biological family wants to be involved in his life, and I can't help him with it. I have decided I am going to tell him I know, and it changes nothing, but I need to do this in person, and I do not live by him.
If my mother was still alive it would still be a much gaurded secret.
I personally would never lie to my kids about it. It doesn't make the adoptive parent any less their parent. Eventually the truth comes out, whether you want it to or not. I would tell him when you think he will be able to understand it. Good Luck!