S.L.
I started running when I was 11. I ran three 26 mile races by the time I was 15. My little brother started running when he was 3 years old. He enjoyed it and could run 3-5 miles at a time. Our bodies are made to move that used to be the way of life.
Make sure he has comfortable shoes and if he feels any pain then stop. You can ice his knees after to be on the safe side if he is heavy. I also have a brother who was overweight and ran. He had to wear stuff around his knees. He was 5'8" and about 250. After running at his own pace he lost 50lbs over time. He stopped b/c work and now he's up to 350 lbs. It's the best sport because anyone can do it. Good luck.