DS3 was younger than your DS when he could fall asleep alone, but DS1 is a teen and still needs someone in the room while he is awake. That isn’t going to change.
Is DS taking prolonged release melatonin, immediate release melatonin or both?
Has DS ever tried any other medication to help with sleep either as well as melatonin or instead of?
Some of the things we tried included: a two-way monitor, walkie talkies, various lighting, dream pad pillow, ear plugs, brushing, worry book, tents, doodle book/pillowcase, music, white noise, various apps, weighted blanket, audiobooks, life size teddy, relaxation, meditation. We also tried the elastic band tech technique and an adapted version of gradual retreat. These are all things some people find works for them.