Your school gives "letter" grades in 1st?

Have you contacted his teacher and asked what is up? This should the first thing you do. I usually go to the teacher and tell them I don't understand or I'm confused. Could they please explain what is going on and give them at least one chance to explain. Perhaps your son is just not completing the assignments for a number of various reasons. Or in not turning them in. Or refusing to do them because it's too easy. Any number of reasons.