I am a professional photographer using a MBP. I have a 2017, 16 GB ram,
3.1 GHz Quad-Core Intel Core i7. For an enthusiast photographer it is more than enough power. It would be enough for the next 3-4 years. I would recommend looking at a reduced priced last year speced 15" MBP, or even a used 2017 top end model. Both would be around your price point. If you want to get a this year model, I would get the 15". Photoshop uses more processor compared to GPU. In fact it wasn't until a recent update Photoshop/Lightroom could use the GPU and CPU together well. If your son is planning on doing some heavy edits, he will need a good CPU, which is on the 15".
This year model: 2.3 8-core Processor (most Photoshop benchmarks don't show much difference between two options, but the 8-core is the better option by far), 16GB ram (Photoshop is a RAM hog), 512GB SSD is enough.
Other things I recommend. A proper external SSD. I'd go with a MVME enclosure and MVME. a Decent online photo course. Figure out what photography interests him. Find a good course. School teachers are amazing resources, but tow teachers are always better than one. Get him an ipad or wacom tablet. If he can learn to use this early, it will be much better for down the road.
A lot of people want to be photographers, but if he wants to get into the industry, good editing skills will set him apart from everyone else. If he has good editing skills, he can make it as a photographer and/or as an editor. I wish I put more effort into learning editing earlier in my career.