Can you explain your situation a little more?
Do you live with him?
Why doesn't he know?
How does the mother feel about it?
How do you know for sure?
Are you asking what you should/could do emotionally or legally?
I need to ask you a couple more questions
How do you know he is your child for sure? Has there been a dna test?
Why doesn't she want him to know? is it because he thinks her husband is his father?
How do you think he will react knowing that the other man is not his real father after 9 years?
Thank you for that additional info Jr. If he knew you as his father from age 4, there's a good chance that he may remember you as his dad, but became closer to his stepdad after you weren't around. You hvae to let her know that she can't have it both ways, she can't ask you for your help all of the time, yet not allow you to communicate with your son as his dad.
I would talk to her a little more, let her know that you are willing to work with her, but you want to know when the two of you can sit down with your son. Depending on how close he is to his stepdad vs how close he is to you will determine how upset he's going to be. If he really loved his stepdad, and he feels like his stepdad is rejecting him, it could make him feel terrible. Divorce is already a very difficult thing for a child his age to process, they tend to blame themselves and think that everything is their fault, or that there is some way they could have prevented the breakup, despite how silly it might seem to others.
Spend time with him, be the support he needs right now. Try to talk to the mom to come up with a plan on when you can sit down and talk to him about it, preferably with her there. If she's not willing to give you a timeframe, I can't tell you to go against her decision, but if you decide to tell him, be sure to look at how he's feeling about the whole situation. If he's really upset about everything it might be good to wait a while.
I welcome your thoughts, let me know if you want to talk more