I agree with legancy on this one. What I often do after my normal shooting is move my target to a new location and walk a few paces away from the target then just turn around and shoot from diffrent random spots(unmeasured distances). If you shoot 20, 30 and 40 yards alot becomes easy to judge after a while (almost second nature). If you're going to be putting him on stand this fall I'd suggest giving him "landmarks" to judge distances by. In the woods 25 yards often looks a lot longer than 25. If you measure out to a certain tree or rock and you can say for sure that tree or rock is 20 yards judging beyond that it easier. This is what I did when I first got into the sport.

Hope ths helps some.