Does your son participate in any math competitions or in a math circle, mathcounts, AMCs etc..? Does he have any desire to qualify for AIME.

AoPS number theory is a great course, as well as their AMC 10 and 12 classes.
The Problem Solving Series Vol. 1 and 2 are good books. He will recognize some type of problems and scratch his head on some.

If my son had to repeat a math course he would be a bear to get along with. Combine that with the reality he has some teacher coming up that are at best uninspiring.