how to tell an older child hes adopted

I am a mother of 2 bchildren and 1 achild. I am looking for some advice on how to tell my adopted son he is adopted. He just turned 9. We have had him since birth so we are the only family he knows but I know it is time to tell him and I guess I am scared of his reaction and honestly I'm not sure how to tell him. Has anyone been through this at this age? Any help/advice would be greatly appreciated.