Best method of exercise for a 12 Year old.?

20 Kgs will tend realistically to take 20 months to lose. A 12 year old male who is 176.25 pounds and is 5 feet and 6 inches tall has a body m index of 28.5, which is over the 95th percentile, and would indicate that your nephew is as you say, significantly overweight. Both dietary changes and regular vigorous exercise will be needed if he hopes to stand any chance of correcting this problem. Any exercises will help, they all burn calories.