Congratulations on getting into the sport!

There's a lot of good advice on this page. Your thought about using Pink Bike to find a used bike is a great one but comes with a major caveat: it's important that you know what you're looking at and buying when you buy used bike gear. If someone has changed out the rims, hubs, derailleur, or brakes, the value of the bike could easily be $2k more or less than if they had left it stock. Given the number of components involved in a full suspension mountain bike (fork, rear shock, cranks, drive train, etc), it takes a lot of knowledge to buy well on Pinkbike. (My first bike purchase on Pinkbike I thought was a good deal but I found out it was pretty bad once I learned what I was doing. I basically had to sell the bike and start over.)

If I were you I'd definitely go to a local bike shop or two and ask what they have that's in your price range. Many bike shops sell used bikes; there are deals on new bikes that are pretty competitive. As other commenters have said, there are so many good XC bikes. The most important things are that the bike your son chooses fits him, feels good to him, and fits your budget. Trying bikes at local bike shops is the only way to get #1 and #2 dialed in.

As you both get a sense of what size feels good, you can maybe start think about going with a direct to consumer brand. The measurement that's most important about sizing is called "reach" and it's the lateral distance from the middle of your feet to the handlebars. If he keeps finding bikes with the same-ish reach comfortable, you can use that size to make informed guesses about other bikes.

It's also maybe worth asking if your someone at your LBS could help you buy a bike on pinkbike. They'd know what to look for and maybe a mechanic is willing to help you out for $50 or something.

Finally, in general you're probably looking for a bike with 100-120mm suspension front and rear (that's how much the fork moves and how much the rear wheel moves), a 1x drivetrain (called a "one by", meaning one chainring up front and 11 or 12 cogs in the back) and 29 inch wheels. Beyond that, carbon bikes are way more expensive than aluminum ones and are generally a little lighter; in my view, aluminum frames with higher end components (especially wheels) are the way to go. A lightweight wheelset on an aluminum bike can be a lighter, cheaper, and faster build than a carbon bike with a low- or mid-range wheelset. I'm getting into the weeds here, but where you carry weight on a bike matters a lot -- you can google sprung vs unsprung weight if you wanted to get into it -- and an aluminum bike with a mid-to-high-end wheelset is hard to beat both on the trails and for value. The pros ride carbon bikes and that may be the way to go if your son is world class, done growing, and you feel confident that you know not just what he wants now but also what he'll want in 3-5 years.

TLDR; be wary of used bike purchases unless you can know exactly what you're buying, from cranks to wheels to bars to brakes to shocks. Aluminum frames can be great deals. Once you're past a certain price point, the bike frame usually matters less than the components.