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.
Answer:
To calculate your target heart rate, first estimate your maximum heart rate by multiplying your age by 0.7 and then subtracting that number from 207. Next, determine your resting heart rate by feeling your pulse after you wake up in the morning and counting the number of beats in 1 minute.
Explanation: