I have started photography not much older than your son is, and then stuck with for 20 years or so. I tried my hand at most things, from fashion and events, to travel, nature, architecture, landscape, scuba, street photography and macro. And a ton of editing of course.
I had a total of four cameras and 4 lenses over 20 years. My budget has never been too high, and at the beginning was really tight.
Still I managed to select what I needed and get it for a good price, with very few regrets. This includes my very first camera, which was cheap and extremely versatile, and very carefuly selected.
I managed because for Xmas I bought myself not a camera but two decent manuals:
1) a manual about photography. It covered all the fundamentals to use a camera with manual settings, a variety of compositional tecniques (composing by framing, by colour, by balance, by using depth of field) and of applications, with examples and what the technical (e.g. skills) and gear requirements were. Explaining also how you could get around the gear requirements, but not so easily the skills requirements.
I choose the manual myself, after going through so many at the local bookshops. I selected one that was very clear to me, it had illustrations I understood and a style I liked. I discarded other manuals which were were highly rated, just becasue they did not seem as clear.
2) A manual about editing, which came with files to do the exercises with (useful because I did not have a camera yet, and anyway I would not have been able to take photos with similar characteristics and issues to work on).
By February, I had gone through the first manual three times, and I had done all the exercises in the second.
And I bought my camera, a pretty good and cheap one, which served me well for many years and so many different applications.
I knew what was not negotiable and what was dispensable, because there were ways to get around it, with some effort, preparation and practise.
And nobody who looked at my photos and liked them ever guessed than my gear was not the latest from Nikon/Canon.
I did get some "You must have a good camera" though, which is a bit like congratulating a chef for the food with "You surely must have great pots".
If anything, when I took a photography course and showed my portfolio, the instructor showed it around saying "See, you do not need an expensive camera to take good photos".
Which is true. It is the same you see often here. When someone will ask about some specific woodworking gadget, there is always someone suggesting you can get the same results with a bit of ingenuity and some scraps.
I hope I am not coming across as a bit of an ass by saying that, it is not my intention.
I just mean that your kid armed with knowledge, determination and a 20 years old camera he understands well would probably take way better photos then your kid armed with 20K of gear but little understanding.
The only exception being videography perhaps, a 5 years old camera then would be a more reasonable contender.
So, if I were you, I would encourage him to clarify to himself what he will need. Meaning, if Nikon/Canon brochure tells you that the latest and greatest has 7 millions focus points or whatever other claim, what does that really entail for a photographer?
How would he get around that with a different camera? Will it make a real difference, for an experienced photographer?
In my experience, most of these things are highly negotiable, with some dedication and knowledge.
Then you can talk together about options, given your budget, and he will probably be able to suggest some specific models he researched about, and why they would be better than others.
Ideally he should have very clear in his mind what can be done with a camera in full manual mode, that helped me more than anything.
In the end, how well controls (ISO, aperture ans shutter mainly) were placed for my hand to operate them all at the same time while framing was definitely a more important consideration than any automated or semi auto features.
And, well.. a concession I have to make to some modern designs is that a really versatile swivel back display will allow you to take photos from very hard to frame angles, at times. And that will make for better pictures than high end image quality, it can make for images which people will actually like better.