Hi ann, sorry you're having a tough time. Those early weeks and months after placement are really difficult for many of us. Please forgive me if anything I'm about to say seems irrelevant or not right for you - obviously I don't know how worried you should be.
But one thing is for sure: he is a traumatised child. Please don't underestimate the impact of what he has been through. And I'm emphasising this because, despite all the prep etc, I did, and other adoptive parents have told me they did. I think we tend to travel hopefully - perfectly understandably - and conventional parenting approaches are hardwired in us, so that's what we tend to do.
So, he laughs when you tell him off. He's defiant and aggressive. This is likely because he is traumatised. He doesn't feel safe and he doesn't trust you. His experience is that grown-ups abandon you, so why would he allow one to control him? This is really normal, I'm afraid - but take heart, as the overly compliant child can be even more troubling in terms of building attachment.
Have you done training or read books on attachment-based parenting? I think you may get a lot from the PACE model or similar. The conventional approach of telling off is often very ineffective with these children (and I speak from bitter experience).
As for being unable to amuse himself, I'm not sure that's that unusual at 23 months. Toddlers ARE exhausting. Even my birth child wasn't self-entertaining till much later than that. And a traumatised child will not feel safe enough to explore mentally or creatively. My 6 year old adopted dd has only just started independent play, I'm afraid, and is still hugely attention-demanding and controlling.
Drugs and alcohol in utero are always a worry but really that just gives you a level of uncertainty that most adoptive parents have to live with. I know my child was exposed to both in utero - she was born addicted - but I still have no idea whether that has resulted in problems, or whether her difficulties are about something else.
Lastly, Ideas on how we can stop this behaviour please? Here I might sound harsh, but bear with me. There's a book by an adopter called Sally Donovan (which I recommend) where she starts by criticising adopters who turn up at parenting courses asking for someone to basically fix their child. I was that parent, still am sometimes, even 5 years in and knowing that there is no 'fix'. In some ways, things will just get better naturally. Your son will get older and not be a toddler any more (boy, are they taxing). Your love for him will grow and parenting will feel less like a bad babysitting job that never ends. His love for you will grow and he will be more motivated to please you.
But you would also be wise to prepare yourself for the possibility (not inevitability) that your son will have lasting issues to manage, because of his early life experiences and traumatic losses. I don't know what kind of therapeutic parenting training you have been offered, if any, but this would be a good time for you to do some. (Especially as it is often easier to access resources like this before the final adoption order.)
The first year is really, really hard. It is very challenging to develop empathic parenting with a child who is defiant and aggressive, but you will find the future much easier if you do.