I am an adoptee and I think honesty is the best advice. Give him the info when he is ready to hear it. You shouldn't lie, but you can always downplay the information you know. My adoption was closed, but my parents were honest about everything they knew. I have had 2 successful reunions with the support of my parents. The fact that you are being proactive leads me to believe that you will not screw up royally
Remember, no parent is perfect, adoptive or not. Follow your instincts when the time comes, some things are better left unsaid.