I think your DS sounds VERY good in math! I think it'd be entirely appropriate to talk with the teacher. I did that last year when DS was in 1st grade and his teacher did start giving him 2nd grade worksheets to do at home. That's all she could do at his school because they don't have any gifted groups or any other groups that get pulled out of the classroom. But perhaps your school does provide those. My friend's DS gets pulled out for an advanced math group 3 times a week at his school.

By the way, if your son loves math and wants to work on math at his own pace at home, there's a math website that I recently paid to join--it's www.ixl.com. Both my 5 and 7-year-old boys have been enjoying working on math there. (It has levels pre-K to 8th grade.) You can try around 20 sample problems a day for free.