Math class is very different than competition math. You need a strong foundation in classroom math for MS and HS competition. But the best way to get good at competition math is to do practice problems. It is possible with tailored math class that he will have time to do some competition prep during class. My kids did a little AMC prep in school.
What is the MS math club structure at his school? Schools that produce Mathcounts National Winners have students who spend a lot of time studying math. Some schools support this with math clubs that provide hours of practice. Others have students who study on their own or have tutors/summer programs/other out of school resources. Kids who win Nationals for Mathcounts often don’t invest a lot of time in other extracurricular activities. One could still play an instrument, but probably not in a competitive audition based orchestra. Same with sports - local sports with one game and practice a week is one thing, but not a club travel team that requires a lot of time.
AoPS is very popular for competition prep. They have semester long classes, and focused weekend classes for competition prep. They also have free sessions to review AMC8/10/12/AIME a few days after the test.