I think being adopted may make some difference to development, but I don't know whether that might make it more or less likely that he may catch up.
Global development delay is a huge range. It could be that he will suddenly start improving, or if he stays at one year behind, at this point it looks huge, when he's fifteen it will make far less difference, or unfortunately it could be that he'll get further and further behind. I'm not sure even doctors can say this.
Down's syndrome often can be guessed at by features, I'm sure you know the facial ones, but there's others like only having one crease across the palm (although that isn't a given-I have a friend who has that).
However there are multiple other development delay genetic causes that aren't Down's, so I'd guess they're checking for others too.
Things I'd ask the paediatrician about is things like:
Muscle tone. Is he floppy? Hypermobile? Tight muscles? What can you do to help? Swimming is often a good one, but are there exercises you can do.
Have you checked (as much as you can at that age) hearing/sight etc. If he's not hearing then he will be later talking. And not hearing can be glue ear which isn't permanent.
What else can you do to help? The doctor may say it's a wait and see, or they might put him on the list for speech therapy (may sound silly now, but there can be a huge waiting list).
How is his eating? If he eats poorly does he struggle with lumps? Again that can be down to muscle tone. Can he get a piece of bread in his mouth when he's holding it? (not all children try to do that, so don't worry). Does he open his mouth at the right time when you bring a spoon near?
Things like that show a little understanding and memory.
But whatever happens he's still your darling little boy and it's tough waiting to find out, but well done for pushing for answers.