Get in and among the crowd in the junior rifle club and find out what is being used. When you wrote "junior rifle" I immediately returned to my days as a member of the Davey Crockett Junior Rifle Club of Ft. Myer VA. In that club we shot serious 50ft. indoor four position. Everyone had target models with peep and globe sights, and full regalia, slings, cuffs, 10-X jackets. Most of us had middling rifles such as my Winchester Model 75, a few had Winchester Model 52’s, and the envied minority had Anschutz 64’s. I do not recall any turning up, but I suspect a light sporter, even a Winchester Model 52 sporter, would have received a lot of razzing. If your son is headed into the snowbird and pinwheel crowd, an autoloader is not appropriate and may not even be allowed. Indeed, one of my most ignominious moments on the firing line was the very public, very thorough, chewing out I received for loading a magazine.
edit: ooops! I suppose it might be obvious, but bolt action was really the only option at the DCJRC.