As a rising 10th grader he needs to be playing 15U with the other rising 10th graders, age is irrelevant once you get to high school and beyond. You need to play with your graduating class.
As for the rest of your post, what are his aspirations within the game? Does he dream about playing beyond high school? Does he do more than dream, does he work on his game outside of organized practices. Does he eat and lift to get bigger and stronger to improve his performance? Knowing those answers will influence the guidance you will get.
As to the struggles, we put our kids in youth sports to gain life skills and learn to deal with adversity. I would tell him simply that baseball doesn’t always love you back. Every player at some point is going to struggle and many will quit as a result.