If a guy keeps saying " be good " to ya but quite sure knowing that he doesnt wanna be your bf, what possibly does he mean in this way? If it is just a friend, he shouldnt telling me "be good" all the time.
He might want to be your big brother (not just a friend), so he doesn't want you to do anything wrong. You didn't give enough details to tell about your relationship. Still another possibility is that this is his tag in every conservation.