AITA for wanting a divorce because my wife doesn’t want her son to be legally mine

I’ve been in her sons life for the past 4 years since he was 2 , he already calls me dad all that’s left is to make it legal, but my wife thinks that decision to make it legal should be up to her son but wants to wait till he is old enough to make that decision which I think is completely silly right ? She also doesn’t want anymore kids which I’ve come to terms with. I’m at a loss now I don’t know what to do. I also heard from her friend she doesn’t want to make it legal because if we divorce she doesn’t want to have to fight me for custody. Edit: she changed her mind about wanting more kids because of her career I accepted it and we compromised on letting me adopt him but now she’s stalling .the bio dad has never been in the picture, she doesn’t even know who he is . UPDATE: she has agreed to let me be the legal guardian in case anything happens to her and we will continue to wait till he is old enough to decide if he wants to be adopted. This is not what I want but I’m willing to compromise. Thank you all for your comments.