My grandson went to Camp Perry with his high school JROTC team a few weeks ago and, although only a freshman, did pretty well. I'd like to support him and am looking at the Crosman Challenger PCP air rifle with the new diopter rear sight (this is what the team is shooting). Team rifles are restricted to use only at certain times so his own rifle should allow more practice even if it is outside at home. Is there anything else a young shooter could use to improve his shooting? The only thing he uses through his team is a palm rest, no gloves, jacket, or shoes. This is getting expensive but he is the future.

Get a scuba tank. Don't even mess with the pumps.
I won't +or- choice in rifle. The best rifle out there, won't make a mediocre shooter better. On the other side, a turd of a rifle will hold a good shooter back. From what I have seen, most club guns are bought on price point. I would rather have 1-2 real good , rather than enough for all and a spare or two. Makes it hard on the kids, but results on paper show what it is worth.
If your going to buy, buy the best you can, if allowable per rules and such. FYI, get ready for jealous folks to make comments. Pay them no mind!
This of coarse is all related to how good of a coach you have.

I'm not up to date on the course of fire or equipment rules used in precsion air rife since I was involved in the 70's. One thing I'm sure that is needed is a spotting scope and stand. You can't call your shots if you don't know where
your hitting. You don't need more than 20X since they are probably shooting 10 meters and maybe out to 50 ft. on some practice ranges.