Actually, the use of a rangefinder is important with either of these two ideas. I've used them with my stepson with good results. He's sixteen. First, start at your target and step back. Range 20 yards, put out a flag, do the same from 30 and 40. Have him shoot at these known ranges. Do it until he's comfortable shooting from these ranges. Then remove the flags and start shooting from around the area.

The other option is to go stump shooting. Again take a range finder. Sometimes range the distances before shooting, and sometimes after if he misses it. Repeat these exercises until the rangefinder is no longer necessary. Good Luck!