I'm not sure what the answer is, but my advice is to keep insisting on trying to find an explanation and also a solution because he is so young and this (obviously) impact the rest of his life.
When I was in high school, my teeth were terrible. When I was in college, I started having some weird symptoms and the doctors diagnosed Lupus with Sjogren's Syndrome (dry mouth, dry eyes, dry everything). The lack of saliva caused cavities and probably now, at 32, about half of my teeth are crowns. I'm not sure, but I think your son is too young for Sjogren's to be a possibility.
Having crowns or implants is such a pain because you have to care for them more than you would your real teeth, plus you will probably replace them at least once during your lifetime. I am looking at starting to replace crowns, and THEY ARE EXPENSIVE!!! Particularly when you have a mouthful that need replacing.
Good luck.