There are many great options for a first time interchangeable lens camera for someone in high school. My first question would be, have you asked your son if there is one that he wants? I know when I was in high school and I asked for my first DSLR, I had already done extensive research on what was available and what I would like to have.
That said, here are a couple options that would work:
Canon Rebel T6 w/ 18-55mm kit
Nikon D3400 w/ 18-55mm kit
Olympus OM-D E-M10 MK II w/ 14-42mm kit
Fuji X-A5 w/ 15-45mm kit
The Canon Rebel is a classic entry level DSLR. I would bet the vast majority of photographers started out with one or have owned one at one point.
The Nikon is a very good entry level DSLR from a very good company. Both Canon and Nikon also have a lot of different lenses available, both new and used, that should work with those cameras. That's a bonus.
The Olympus I really don't have any experience with, but Olympus has earned a very solid reputation over the last decade or so with their OM-D series of micro four thirds cameras. Micro four thirds typically pack a much larger sensor into a much smaller body than comparable cameras, which helps with image quality (larger sensor = more light captured = less noise = = better images).
The Fuji is a brand new model that's yet to come out, but it just recently announced. The X-A line is their entry level line of the X series mirror-less cameras. I personally shoot with a Fuji X-E1 (which is discontinued now, and I'm sure has been bested by this "lower level" X-A5 at this point).
Mirror-less cameras and micro four thirds cameras are interchangeable lens systems, but they are not "DSLR's". The Canon and the Nikon are true DSLR's. That said, I (and many others) have switched away from DSLR's, at least the pro level ones, because they are very large, very heavy, and do not really offer any additional image quality over the smaller, lighter, and more compact mirror-less offerings from Fuji, Olympus, or Sony (which I did not include here because they are rather expensive, but are very good).
At this point, for a high schooler who is used to shooting with a phone or a point and shoot, any of those four would be a vast improvement, but I'd still suggest asking if he has maybe already picked one out or done some research on it.
Another piece of advice that is always given out to someone who is looking to buy a new camera is to see how it feels in your hand. Back when camera shops were common you could go and actually hold the cameras before you bought one. Some just don't fit your hands as well as others, and the saying goes that if it's not comfortable to shoot with it, you won't shoot with it. That advice is a bit harder to follow now that it's difficult to find a place to hold a camera before you buy it.
Let us know how it turns out, and maybe have him sign up for the forum and share some photographs.