Honestly, I would tell him that (if he plays sports or plans of doing it) the smoke and carbon chemicals that he is inhaling in his lungs are going to inhibit him from ever being able to do that. Also, he needs to understand that there are many health problems that can come from cigarettes are devastating. He could get lung cancer, lip cancer, liver cancer, etc. I hope this helps you.
I would explain the bad things smoking does to you and your body. Since he's still a bit young I'd be easy with him. I would probably tell a trusted adult as well.
