Sorry to be blunt but he was rightfully denied. USCIS is discouraging AOS from visitor visas since this is abused mostly (by desis and Nepalese too). You should have let him go through consular process like everyone else. I don't see any reason why appeal or motion to reopen will favor you. Just follow the rule like everyone else and you will be fine.

https://www.visajourney.com/forums/topic/535005-denied-aos-please-help/