In my pre-K class, tattling is a HUGE problem. I also tell my class something similar as your son's teacher except I let them know that it is alright to tell if someone is hurting themselves or others. There is a fine line between tattling and reporting, and it is difficult for little guys to differentiate. Plus if tattling is an epidemic in your son's room, the teacher might not appreciate even the "reporting." The best thing you can teach your son is to encourage his friend to tell the teacher, and even go with them to get attention for their injuries. Then he is still being a good friend, is going along with the teacher's wishes, and is also encouraging his classmates to take care of themselves.
Also, talk to the teacher. She might not realize what he is doing, or he may be tattling and just not seeing it that way. The important thing is, his heart is in the right place.