Hello and welcome 
Pleased to 'meet' another person adopting a child in double digits! My eldest daughter was 10 years old when she came to live with me. Spent 4 years in care before coming to me, including one adoption disruption.
First of all, how well is he being prepared for this? It's very unusual for a child to be adopted in such a situation as your son. Of course feel free not to answer on a public forum, but it is essential that he's had some life story work done and he has been prepared for this latest move. Even better if he's had therapy or other CAMHS sessions etc. He's been through so much confusion and I'm not surprised he's struggling to make sense of any of it.
My DD1 just could not believe that I really was her 'last mother' for years and years. I'd say she was about 16/17 before she truly felt secure that I would never stop being her mum. Attachment, trust, feelings of safety..these are all things that can take a very long time to develop with an older child, of course also depending on their history, amount of moves etc. He has no reason to believe that this placement is not going to break down in a couple of months or a couple of years, and there's nothing you can say that will change that. Only your actions will mean anything IME.
Intros with an older child and setlling them are really very exciting and daunting! I was terrified the first time I met DD1, I kept imagining that as soon as I left, she would turn to her FC and say "I don't like her, I don't want to go and live with her!" She didn't! But still, there's more pressure when your child is old enough to have opinions on you!
I wrote a blog post about intros with an older child and tips, based on my own experience (I adopted a second older child when my DD1 was 18, DD2 turned 8 during our introductions) - if you want to have a read, it's here