It sounds like the issue is that he is worried about what is going to happen, and it sounds like there have been some inconsistencies.
He is 12, so barring SN will have good comprehension.
I would try saying 'yes, DS, when you wake up, you WILL be alone. BUT as soon as you wake, you can call me, and I promise I will come STRAIGHT through to see you.'
They say that the reason babies and toddlers can't self-settle during the night, is because the parent is there when they fall asleep, then gone when they wake.
So, you could start by agreeing with your DS that you WILL go straight through, day or night, when he calls you.
Then, once that is consistent, you move to saying 'yes, you WILL be alone when you wake, but I PROMISE that 1 minute after you call, I will come straight through.'
Then, perhaps, 5 minutes...
Then I suspect that one morning, he just won't call.
Also, give him a proper light, so that if he wakes, he can read a book, etc.