So several tips here.
1. The issue isn't the camera, it's the lens. Almost any DSLR or Mirrorless camera can shoot macro photography--with the right lens. But it's the lens that allows you get up close or to magnify the subject. So you may end up paying more for a macro lens than you do the camera body. Let me explain. I can put a zoom lens that will have a focal length of 70mm. But the closest I can get to the subject is 3 feet--any closer and it gets blurry. But if I put my 70mm Macro lens on the exact same body, I can get up to 6 inches away to shoot.
2. If you can supply an approximate budget, that would be helpful.
3. Keep in mind, your son may be interested in macro photography right now. But if he enjoys shooting macro, it may very quick branch off in to shooting....wildlife. Or landscapes. Or concerts. So while you're buying something to shoot macro with, having flexibility is a good thing.