Agree with this but I also think you need to talk it over with him, not at bedtime.
explore with him what going on - not a bedtime! Some other time when you are both in a good place.
not ‘why won’t you go to sleep on your own’
and no ‘big boys sleep on their own’ shaming isn’t going g to help.
try ‘I know you like to have one of us there. What bothers you if we’re not there?’
see what he says. Try the book The Explosive Child for good conversation guides.
what is he like during the day? Is he ok being alone / and alone in his room then? Is he anxious in the day or is it just a bedtime thing?
once you have a sense of what’s going on for him in a second conversation say ‘the current situation isn’t working for me, I need some downtime too. So we need to work together on changing it in a way that works for us both’.
to the above poster it sounds like waiting for him to sneak out isn’t working.
so a change to being clearer that you’ll leave and come back to check on him is a good idea.
id start with building that up - can he manage 5 mins alone, 10 mins, 15, 20, 30 over a few nights.
i agree listening to audiobooks is a good idea. Nice gentle stories though - I had one like this and if the story was too scary or exciting she’d lie anwake for hours listening to it.
anyway after he’s done his time himself then after that time you’ go back sit with him while he falls asleep.
have you got the book The Rabbit Who Wants to Fall Asleep? Sorry and hypnosis.
id also try giving him something like valerian or cytonight to help him fall asleep more easily while you are doing this and reset his habits and beliefs.