I think my husband is probably not the biological father of his son
Why do you think your husband isn't the biological father?
His ex was a slut and that fucking bitch probably slept with other men while she was with him.
If she cheated on him, that was wrong of her. If you feel that their son is not his and you are taking on a stepmom role, then this could be something to talk to him about. The only way to know for sure would be a DNA test.
I don't know if he would want a DNA test. I don't think you understand how it feels for a man to find out that his wife was sleeping with other men.
Do you think it's better for him to find out or just leave it in the past? Would it change anything if he found out the child wasn't his biological son anyway? You should consider if he would want to know and how he would feel before making a decision.