I've name changed just to reply to this post, as I think you're getting a harsh time OP and my previous posts and what I'm about to say here would be quite revealing.
Anyway, in short, I'm in the same boat. I have an 8yo DD, soon to be 9 who thinks someone else is her dad. Her biological dad decided, at the age of 23, that he wasn't ready to be a father. As I was already a single mum at that stage to a 3yo DD I knew I could do it alone, so that's what I did. Her fathers mum (paternal grandparent) reached out to me just after DD was born to say she was devastated this was the decision her son had made, but she had to respect his wishes and basically said her goodbyes. There has been no contact since.
I met my partner when DD2 was 11 months. We moved in together after two years, but DD2 is the only man she has known. Due to us not living together when she first started to talk, she does actually call him by his given name, and not dad, most probably because that's what her sister calls him. Her older sister knows her dad, but also doesn't see him either so both girls have been raised by me, and then by my partner when he moved in. I have noticed however that my DD2 does refer to him as "dad" when talking to other people about him, particularly her school friends.
DD2 has never asked why she calls him by his given name and not dad. And I think until she does, we won't get into the detail. My DD2 is a very young 8. There is no way she would grasp the facts of life, or processing that her biological father was too young. Her elder sister knows that my partner isn't either of their dads, but has never shared this with her sister.
So in short, I think everyone saying you've lied, you're creating issues may have their own experiences. And that's fine. Alls I know is for now, it is not the right time to tell my DD2. She's fine, she's loved, she doesn't have a family out there ready to embrace her as they've never reached out. I know my daughter inside out, and I know now isn't the right time. And I believe only you will know what's best for your son, no single piece of advice on here will be a best-fit option 