You’ve had a very hard time here and agencies haven’t helped.
I would say though he’s very young, how does he understand adoption and what that means in terms of him and his future. The legality of adoption is something hard for many adults to understand much less young children dealing with all the trauma and loss they’ve experienced.
Would you disrupt the placement if you didn’t formalise the adoption or would you continue under a permanent foster placement? Does he think that if he’s not adopted he’ll return to his birth family - which wouldn’t be the case. Or does he fear that adoption will mean the loss of contact with his siblings? These are things you can explore with him.
In terms of decision making, if this was a serious health condition would you leave the choice of treatment up to him, or would you as parents listen to his views and then make the best decision for him even if that wasn’t what he wanted? The reason I ask is that you seem to be basing your decision purely on what he wants, we don’t leave life changing decisions to children, we listen and make the best choice in their interests. At 7 I wouldn’t expect him to decide about being placed with siblings, or whether he wanted to be adopted. I expect him to have the situation explained to him, and to have been asked his view but the decision making rests elsewhere - it’s too much responsibility to place on a child.
He may not want a new family out of loyalty to his own family, or because siblings don’t have a “new” family if they have remained in care, his understanding of what it means to be a family may be skewed by previous experiences, he may also be very scared of being part of a family given his first family wasn’t a safe place for him and then broke down. The reality is you’re caring for him, loving him and parenting him which makes you “family” regardless of legal status, how does he see his relationship with you and your partner?
It’s very difficult to know what to do for the best. What are social work saying?