moi aussi I am not a therapist/psychologist and have no knowledge nor training in this field. The only reason I said to give him a phone is so you can contact him on the island, to verify that everything is okay.
Has he been checked out by a doctor to verify that this problem is not caused by a chemical imbalance? Have you or anyone else spoken to Rabbium about a yeshiva that will cater to his needs. They know the boy and are better suited to find a yeshiva that will meet his special needs. His weighing 48 kg means he lives outside the US, and is 106 pounds, I was 125 ponds when I got married 34 years ago so he is not so underweight. I am now 185 lbs and the doctor wants me to lose weight so that is not a problem.
Have you tried giving him ice cream. You may have a problem with choosing his favorite flavor. I don’t know anyone that doesn’t like ice cream. This will put him in a happier mood and increase his weight. Also try to have him listen to happy music, this too will put him a happier mood. Try taking him to an amusement park, with exciting rides, this will show him how people interact while having fun and may awaken his social skills.
Please check with your medical provider who knows the boy before taking my advise. Likewise you may also want to seek the advise of a Rabbi who knows the boy before following my advise.
I hope it works but it is going to be a long process to get the boy to social interact so don’t give up hope.