Thank you for your question!
This sounds like a good idea to homeschool your son in math. The beauty about homeschooling is that it is flexible and easily adapted according to your son's needs. If he can do math well on his own, this way he can go at his own pace. I would suggest getting Saxon Math 6/5 for your son so he can work on it independently. If he is advanced, he can work on it independently. When he completes the book, perhaps the school will be willing to test his achievement level, and based on the results perhaps advance him in math. As far as the credits are concerned, you will have to address that with the school. Again, the advantage of homeschooling is that your child can advance at his own pace and not be held back based on the abilities, or lack thereof, of his classmates.
God bless you and your family!