If the school has all maths at the same time (or whatever subject), you might choose to propose that he be placed in 3rd or 4th grade for math class. Then, have a list of ways that this would benefit your child, such as developing study skills, developing the need to pay attention in class, developing the ability to get along with children of different ages (which can be related to a real life need in the workplace and elsewhere), and showing your child that school is the place where learning occurs.
Even if they are open to this idea, be prepared that they will want to do a lot of testing. Some schools will test for end of 3rd grade math and have a reasonable expectation of 85-90% mastery to move on. Some schools want your child to fail and will say he needs to get 100% to move on.
Some schools will get really caught up in this idea that it is unfair to all the other children if you child has advanced teaching. I would be ready with an argument that all kids ought to be expected to be learning new things at school. They also may push back by saying that your child MUST behave a certain way to get the advanced materials.
There are other options such as taking pretests so your son can do alternate projects instead of having to pretend to relearn ideas he has already mastered. That requires a very dedicated teacher to do it well.
If the school is very negative, you may prefer to see if there are other schooling options. I hope this helped.
