Trek bikes are very good. My son is a mountain bike fanatic and he has one. He is constantly 'upgrading' parts on the bike. They do get a fair bit of abuse!! His advice is pay for the best quality possible on gears and brakes. Don't know about any bargains though. He is not here to ask at the moment, but I think he bought his seconhand.
You as a parent are about to enter a very dirty world. Be prepared for alot of muck and a fair bit of blood. Mountain biking needs alot of paraphernalia - the leg guards, the elbow guards, the body armour, the helmet (with mouth guard), the helmet light with v. expensive charger, the shoes, the clothes, high energy cereal bars, drink container........ it goes on and on. Oh, and don't forget the bike rack for the car, you will frequently find yourself out on the side of a cold wet mountain, in the dark, whilst these maniacs whizz down the side of the mountain, narrowly avoiding trees and rocks. The latest request is for a power washer to remove the six inches of muck off the bike and themselves. Here's a tip: on the way home from the mountain, stop off at the car wash so he can hose down the bike and protect your car seat with an old blanket. Carry a bumper size first-aid kit and learn some relaxation exercises and I'm sure everything will be fine
P.S. Always make sure he has two spare tubes and a tube changing kit, and that he knows how to change the tube. this will happen on two out of every three treks.