At what age should we tell our child he was adopted?

ASAP. Kids that grow up knowing they are adopted take this as a natural part of their life, and have time to integrate the concept that adoption is a good thing and that this is their family. All children have many questions about adoption that should be encouraged and answered. You would not want them to find out via someone else causing trust issues between you and your child.
When ready. Familial relations are complicated, even more so for children. When they are ready to understand the differences, then it's time to tell them the truth. If your child ever asks if they are adopted, tell them the truth. Never lie about this!