My son doesn't know I'm not his real mom...

I game into my son's life when he was 3. I've always treated him like my son and loved him more than anything. He's getting older now and started high school next year. He doesn't know I'm not his real mother. I have no idea where his bio mom is. When h was 1 1/2 my husband came home from work and caught her giving their BABY a BJ.... He beat the shit out of her and then called the police. Her sisters 7 year old at the time had also opened up about my son's birth mom fingering her... It was all disgusting and she went to prison. I wanted to tell him a lot sooner but my husband didn't think he could handle it and now he's about to start High school.... I feel like the longer we wait the worse it will be...