How long has he been in school? It's very normal for the first month of school to be largely review. The question is, does he pretty much have typical 1st grade material mastered. Is he reading? Can he add and subtract comfortably? How is his writing? Personally, I'd give it two weeks and then schedule a conference with his teacher. If you have any proof of higher ability at home, bring it... a book list, a story he's written, math problems he's asked for "for fun." If you don't have anything, that's OK to. The conference is a good chance to find out where the class is going, will there be an ability grouping, etc.
Homework tip to try... don't expect him to sit the full 20 minutes. Instead, break the homework up. He doesn't need to sit for a whole page. Set a goal of half a page. Let him know in advance that after 1/2 page he can go play for 20 minutes. Then bring him back for the other half. It that doesn't work, talk to the teacher about some alternative work. Maybe more challenging material will hold his interest.
Married mom, DD 18, DS 15, and a Valentine's surprise on the way!