Intentional foul and sub?

NFHS rules. A1 is on a fast break, goes up for a layup. B1 intentionally fouls A1. A1 goes down hard, stays down for a minute. Trainer and coach come out to look at him. A1 walks off under his own power to the bench. Coach A asks if A1's substitute can shoot the free throws, and later have A1 enter the game. What are the rules governing this situation? I have searched the books, but have not found anything.