Ok to weigh in here. I went to the end and just starting giving some info so you may have gotten some of this.

He is not really testing anything if he is not comparing anything. If he is just shooting at increasing distances, yes he has one varible, but all he has is the terminal ballistics of one thing, a PRB. What I would suggest (as someone who has a degree in biology and has judged science fair projects) is comparing 2 or 3 bullets with the same powder charge at given distances, maybe 25, 50, 75, 100, 125 yards. Create a hypothesis that he can test like PRB will have the best penetration, weight retention, and expansion of the 3 bullets tested. I like the idea of clay versus sand because I think you can get a more precise measure of depth. An idea is get an 8" by x" longpiece of PVC, split on bandsaw, fill with clay, hold together with string, shoot, split, measure depth, mix clay together, put back together shoot again. The more shots the better but a shoulder and the wallet can only take so much. Don't be bummed if you get a complete pass through the clay but try to make the clay long enough that most of the shots are stopped.

Feel free for more help. Reminds me of a science project that I did on penetration power of steel shot vs lead shot when we had to switch to steel in the early 90's.