Have I just been totally niaive and stupid not to think about that more before hand?
Not stupid, no, but in the gentlest way possible the transition he’s making is incredibly difficult and what you’re describing is to be expected.
While he’s 7, so maybe has a different awareness of what’s happening than a youngster child, he’s going to be very, very confused.
He’s been taken (for the second time in his young life) away from the home he knows and the people he knows to be family to him and placed in a completely new place with new people. He’s been told they’re “mum and dad”, but what does that mean when you’re on your third set of parental figures in the space of 7 years. He’s been taught how to behave and fit in and be accepted in his last two places and now someone has changed the rules. And they sound different and smell different and eat different things.
The poor child will feel like he just can’t get it right. At 7 he’ll have a good awareness of how he’s expected to behave but your expectations will be different, your home culture, the unwritten rules, he doesn’t know any of them and will feel like he’s doing something wrong because the people who raised him to this point taught him how to behave. Whether he calls you mum and dad or not, you’ve not earned the place in his life yet to tell him what to do or not do. What that means is you need to discuss and explain and understand. So if you give him a chocolate bar “let’s both have some chocolate - it’s ok to have a treat every now and again” but also have a “permitted” snack available. His anxiety may be too high to enjoy the chocolate if he thinks he’s done something wrong.
Take the same approach to everything, explain why you do what you do or why X behaviour is ok/not ok in your house. He’s not trying to be hurtful, he literally doesn’t know how the world works so teach him the rules in your house and, unless it’s harmful, let him follow his old rules until he is ready to let go. He will, in time.
It’s hardly been a week, he’s adjusting and so are you both - he’s an older child and this transition is going to be very difficult for him. I can’t imagine how bloody terrified he must be - every single thing in his life has changed, and he keeps being told to break the rules, and his old parents never want to see him again - he may think that’s because he’s done something wrong so be very clear about why he’s not still with foster carers (not in a way that blames them, but that explains they were caring for him until his new, forever mum and dad were found, and now it’s time for them to care for other children who need a new family too).
On a practical level, get some rest and some space - you’ll cope better with a clear, rested mind. While your head knows he’s not rejecting you per se it feels very very rejecting, so take it easy where you can.