Thank you Abba_S for your advice, you must be a therapist/psychologist.
He goes to minyan like a robot. He wouldn’t need a cell phone on the island, his phone is rarely activated. Regarding food, he weighs 48kg (he’s 19), he eats to survive (no joy in eating). I wouldn’t say he’s depressed per se, maybe autistic traits…?
Where does one find a Yeshiva Gedolah for such boys? Most specialized Yeshivos cater for younger kids.