this sounds so hard! i don’t have much to add except that maybe worth getting labs? i’ve heard low iron can really impact sleep
Has he been tested for apnea?
No personal experience to share, but if you haven't tried compression sheets, HugSleep, or weighted blanket maybe it could be worth a try to meet sensory needs. Deep pressure calms the nervous system. Also, my hairdresser's son had trouble sleeping when young and it was something to do with his tonsils. She said it took forever to figure it out. After a minor surgery he was great.
So I have some experience here, my 6 year old is autistic and he doesn’t really sleep much either but it doesn’t really affect him so we just let him be he is really good at not walking out of his room or getting out of bed in the middle of the night if he’s not sleeping. We have stuffed animals for him and quite toys for him in case he can’t sleep then he talks to his stuffed animals or reads a book. He doesn’t wake up his brother either which is 2 years old. My 2 year gets his sleep pretty well and is able to sleep in even though they share a room. I only have experience with my son but I feel like as long has he can entertain himself without waking others up he’s ok and I let him be but I remind him every day that he needs to be aware that others may not want to be awake at that same time and he should try to keep it down and not make too much noise. In our home we live with other family members so sometimes everyone is sleeping expect for him and I.