Sounds simple, but have you asked him what would make him more comfortable (apart from you being around)?
You have my sympathy - been there, done that. I have 3 dc, all with ASD, and they have all had this to a greater or lesser extent.
My eldest had a huge phase when she was little, and like you we had to be there while she was settling to sleep etc - it took about 4 years to get through that phase! I put her and dd2 together sharing a bedroom to try to resolve it (it helped a bit) - is that an option? Eventually (due to some building work we were having done) I had dd1, dd2 AND ds sharing a room - dd1 would have been about 11 by then I guess, and when the work was finished (took a while!), she was actually ready to be in her own room on her own again.
The constant need to be with you during the day can be wearing. I’ve been through this too, and it is so hard when you can’t even have 5 minutes peace to sit and breathe. Try working on this slowly and building it up gradually. I used to get dc settled with doing something (I was doing it too - craft/watching tv, building Lego, whatever) and then ‘remember’ something I needed to fetch or do in another room close by. I said what I’d be doing, where I’d be doing it, and made sure I was gone about 30 seconds initially - I’d often meet dc coming to follow me on my way back! - but I made sure I did what I’d said I was going to, and could build it up (painfully) slowly. I still announce to dd1 (17, has learning difficulties) where I am going/what I will be doing, and still make sure I am only as long as I said I would be. She cannot be unaccompanied for long, and so I am well used to doing all my general jobs with her tagging along (actually, giving the control to her sometimes works - ‘I need to empty the dishwasher, do you want to com with me and chat, or stay here doing X’ - she does sometimes choose to stay doing X.
It is tough, but hang in there, it will improve with lots of support and encouragement (ds, who is 9, is forever up and down all evening after bedtime, as he is too lonely in his room - he gets his couple of minutes sitting with me, lots of hugs and reassurance, and back up he goes for a little bit, before he’s back for more - he does settle eventually. One thing that worked really well for him was a colour change lightbulb - once I’ve said goodnight and done his official ‘lights out’, he’s allowed his lightbulb on dim/a dull colour, and he can then turn it off using a remote when he’s ready for sleep. It’s worked ether than a nightlight/corridor light being on etc)