He's 15. Many adults whose native language is English can't write it correctly, or what are you referring to if not grammar errors? Can you give me examples?
The way grammar works is that it is hard to describe the rules of your native language. I am parsing this as non native. My older kid is a few years younger than this guy, and her text messages to me sound more native than this corpus. No I can't describe it concretely. It's just how I parse word choice and sentence structure.