Little ones learn from the world around them.
Out of the house, people overwhelmingly are positive towards them and he responds with positivity. They don’t have to do the ‘telling off’ or the ‘parenting’ they are solely positive figures, so he isn’t challenged by them.
You however challenge and parent him, you have to do the boring things and the things he doesn’t like. You expect things of him that he doesn’t want to do, and rightly so. So he responds defiantly because he doesn’t like this. And he knows you will still hug and love him despite this.
Add to the mix a ND elder sibling and a new to the house step sibling who shows very challenging behaviour, and all of a sudden there is less attention available from you, towards him. And a lot of that is naturally asking him to do something/stop doing something. Perhaps those siblings don’t respond to consequences so he sees them challenging you or not getting the same consequences as he does - it may seem like there are different rules for him to them.
And knowing a bigger child is punching holes in walls must be really frightening.
It sounds like he is trying to find his place in your new family set up, and testing your boundaries towards him.
Step siblings behaviour needs addressing quickly. Help regulate the tension in the house hold.
Same expectations for all children in the house hold - basic expectations like respecting each other and listening.
One on one time just for the two of you, where he leads, you do fun things, and there are no expectations on him aside from to have fun. Enjoy him!
Perhaps young carers for him, so he spends time with other children who know what it’s like to live with ND siblings? And support for you as a parent to understand how he may feel about this?