Some possible solutions are
*negotiate with the school to allow you to send in math work for him
*negotiate that he does online math during math class (Khan Academy comes to mind)
*the school may be able to do an individual math pullout for him during math class or have someone come in and teach him a separate lesson within his classroom during math class. My son's former elementary did things like this for him by having an AmeriCorps volunteer (who is now a licensed teacher) shadow and instruct him individually within the classroom. They offer subject acceleration within the classroom by having the GT coordinator come into the classroom during math and teach him a different lesson.
Math is SO difficult to differentiate, acceleration is necessary. Figuring out how to make it happen - and within the school's budget and policies - can be difficult.
However, if your son continues to accelerate in math, you may end up in increasingly more difficult situations. I choose to subject accelerate my son and now every year gets more tentative and difficult.
My son was doing 4th grade math in 1st grade, similar to your son but I don't think mine was advanced as yours. My son is 9 years old now, would be 4th grade by age. He is homeschooled, registered as an ungraded student. He's doing Algebra 2 now. :faint: After he gets through Geometry, he will be past what I am able to teach him. I've already exhausted the local virtual charter school - which is wonderful for elementary and middle students but not good for us for their high school policies. I'm attempting to slow his acceleration by offering electives and introducing AP studies. My son's profile is extremely advanced in grammar/reading, age-appropriate writing (so a dramatic lag there), immature, and ADHD. Grade skip was not a viable option for him.
