i think you are right not to refer to yoursefl as mummy. this is not to take away fom your place as her birth mother. its just that small children think very concretely. Mmummy is an ongoing relationship, the person who lives with you and cares for you.
i have a 4yowho understands that he has step siblings who have a different mummy but teh same daddy.when he forgets and refers to me as X and Y's mummy, i remind him that they have a differemt mummy, "remember jane is their mum?" . he jus says "oh yes i forgot" and moves on.
obviously when they are older they can understand that there are legal , biological and relational aspect to being a parent. in some families the same mum does all three. in others, its different. So in our step family i care for my step children but their mum is still their legal parent and they have a biological link with her ( they dont see her).
nowadays there are so many different families i am sure that your son will just accept this all. there will be plenty kids in his class who will have 1/2 siblings and step siblings who they dont live with, but see a few times a year. its pretty similar
I'm sure when he is a 9 or 10 he will ask more questions, but then they can understand more complex things. because teh story is never simple is it? however straightwaorward it seems, there is always a lot of pain and loss for everyone
i meant to say - it says a lot for you that you have kept up such positive contact with your oldest child. I'm sure this will really help her to deal with soem of the issues around being adopted.Her adoptive parenst sound pretty great too!