I'd pay for Art of Problem Solving classes. Since the school is willing to accept partial homeschooling, then they might also be willing to accept the grades AoPS will issue upon request and include them on his transcript. If not, lots of kids do AoPS and then also do school math as a box-checking means to an end.

Also, Art of Problem Solving's Prealgebra is worth doing unless you're absolutely certain your son is beyond it.