When to give hard news?

I'm going to try to make this short and sweet! It's kind of a long, complicated story. My sons biological father is not involved in his life. My son is almost 6, and has maybe seen his biological father 1 time in over 2 years. So he doesn't even really know who he is. We don't talk about him, he doesn't call, etc. My husband adopted him last year, his biological father signed over his rights. So everything couldn't be better! My husband is an amazing father and role model for my son. (His bio dad was a drug addiction and had two other kids). But now we're worried about when our son gets older, and we have to have the conversation about how my husband isn't his biological father, and he has a biological father out there who never kept in touch. I don't want him to feel shocked and upset. I feel like he could either be grateful to have my husband, or feel wonder and anger and try reaching out to his biological father...which obviously scares us. We know kids can be unpredictable and I don't want him to have identity issues, not understand where he really came from. Looking for any positive advice or your own experience that would help us?❤️ Thank you!