Hi slkk that all sounds very hard work and difficult for you. I hope you are able to relax and not take the behaviour too personally, as others have said.
Lots of wise advice above, and I too thought of Lego, as it is a nice toy to make and take apart. I also wondered about things like junk modelling, making stuff and expressing praise when he does it. Appropriate praise, not over the top, descriptive, and at a level he can handle. EG "That box looks really good now you have stuck all those stars over it." etc etc. Nothing too over the top. I also love the phrase, "You must be very proud of yourself." When they do manage to do something good or treat something nicely.
My ds is 4 and arrived by adoption last year aged 3. He has managed very well but is still very emotional, cries very easily, gets upset very easily.
In our training/prep we had the image of the 'fountain'. The water jetting out of the top of the fountain is the behaviour, the main column of the fountain is the belief the child has, and the base (where the water comes from) is the child's needs.
I hope I am remembering that rightly, please someone correct me if I am wrong.
The behaviour is a language to tell us how the child is feeling (a lot of the time, maybe not always but a lot of the time).
So in order to understand the behaviour we need to understand the child's thinking of their needs. It might be they feel unloved, unwanted, confused etc etc. This might lead to a belief they do not deserve nice things or have no value. The need might be for love, shown in an appropriate way they can understand, or for care, or for consistency or order or a bit of all of them.
I don't think being strict or firm will help as it will only tackle the behaviour. It might* stop the behaviour, but if the beliefs and needs are still there then the behaviour may move on to something else.
If you have post adoption support or advice, therapeutic parenting or theraplay or anything else on offer to you locally I would ask to access it now.
My ds has been home just over eight months and is now in school part time. Since starting school he has been much more emotional than he was before. Is your ds in school or preschool yet. If he is this may be adding to his pressure. For me starting ds at school felt like a massive decision and I feel it was right but we are going slowly (he is almost always just half days) and I would say ds is managing quite well, has very good (advanced language skills) and so with delayed language skills it can be even harder for your poor ds to get across what he wants to say.
I hope you get the help and support you need.