I’m a math teacher. The problems are hard. That’s why. Your kid might not have learned all of the topics and it’s different that what is taught in class.

Is he into these competitions or are you pushing him? There are very few kids at my school who really get into them.

Most of these replies are unduly harsh. AMC and other math competitions are not "competitions" in the usual sense but just another exam. The kids are very collaborative and most of the ones that are active on the AoPS forums really like hanging out with each other and do math for the fun of it.

As for the notion that it is too much of a stereotype unless you are a "near IMO" type -- note that there are 6 kids on the US team. 20 or so make it to the team selection test. So one should essentially discount these. If your kid enjoys it, there are several free resources and online forums (much like DCUM but far kinder) where kids learn a lot.

Finally, you don't need to do competition math to do math in college but the kids who make USAMO/JMO find math in college really easy.