Hi Emma,
Welcome!
I felt the urge to respond to this, as your comment reminded me of my time in HS. My response is going to go against the rule of thumb, which is desktops for gamers.
So, there's a reason why I was gaming on a laptop for many years. However, that is not to say that most of the time I didn't have a desktop. I did. I had both a desktop and a laptop usually. It wasn't until I was in college that I was laptop-based for a good while. But, I was also a nomad, sort of.
Here's the main benefit that I had with a laptop --> I was mobile. If I was kicked out of a room, not a problem. If I ended up staying late at school for one reason or another, it didn't matter. If I wanted to game and the family was going on a trip for the weekend, that's fine, I had my laptop.
Here's the downside --> any gaming laptop worth the money is going to be "heavier" than a basic laptop. However, today's heaviest laptops are what the lightest laptops were during my time as a "nomad" (my friends considered me a nomad since I was always on the move). I carried around a 18" laptop that weighed around 14lbs. I think that was the heaviest one I ever carried. Had to look for a certain bag just to fit it. I think the lightest laptop at the time was around 5lbs. Maingear's (maingear.com) most robust gaming laptop is 4.4lbs today.
Are laptops upgradeable? Lol. Absolutely not. The cases are, in general, designed for the specs. You can't just be adding RAM or swapping out a graphics card. However, if your son is young enough, and if you basically get the best possible thing out there, he could technically use the laptop for like 4 years. Why am I saying this?
Before I got my husband @TurboR56Mini into gaming, my concern was always about getting kicked out of my game room/home office. So, if he needed the space, I could just take my laptop to the living room. I ended up also buying him a gaming laptop when I bought my last one in 2015. Our laptops lasted us until December 2020. Technically, they could even still run some games today like Destiny 2, Civilization, Sims, Fallout, Witcher 1 - 3, Mass Effect, etc. But, those laptops were also near-top of the line at the time too. Each laptop cost us about $2800 each. Paid for by year-end bonus. They did their jobs well. I still use mine to watch things if I want to lounge in bed.
Now, if you do go desktop, which is always going to be the best way to go, I would suggest trying to find a quality PC maker to start you off... or a local PC person to teach your son how to build one. These days, I can't be bothered to build my own. Plus, getting parts is a pain. PC builders tend to have parts that consumers cannot get. Since I don't build my own anymore, I'm very loyal to Maingear here in the US. They are a great company. They have fantastic customer service and top notch quality. They work with you if you want to upgrade components. The only reason why my latest PC is from Xidax is because Xidax gave me a killer deal and they had the chips I wanted... Also, another good company.. but they are not in driving distance to me if I want to do a component swap.
Anyway.. just some thoughts I felt like throwing in for this conversation.