10 hours a week is a good amount of time if you are taking previous test (MathCounts or AMC) and going over the solutions. I know the kids around here that make nationals in MathCounts are doing at least 2 full test on their own each week during the season.

The test get harder as you move up so to prepare for State you practice previous state test or maybe even previous national test.

What are the competitions your son has been doing? He has time plenty of time on his side.