How I explain to my son that my husband is not his biological Dad, that he adopted him?

When the time is right you would be best to be 100% honest with your son. Do not bad mouth his natural father but inform him that at the time his dad wasn't mature enough to handle the responsibility of parenthood. You could also at that time let him know how involved his stepfather has been in his life all the time you've been together and that he should continue to view him as his real dad. The boy may eventually decide that he would like to meet the sperm donor who created him, but that should not be concerning as this is pretty normal to want to meet people who have somehow rejected us and to either question them as to why or what you think you did wrong to make them decide this. It will certainly be an emotional time and the entire family will have to bond as a family unit at that time just for the strength alone. If and when he does decide to meet up with his sperm donor you may want to there also, if he is comfortable with this. But if in the future he decides that there is no need to meet this other man then you will have nothing to worry about. Sounds as if you have found a good man to help you raise this son and for him to call dad. Best of luck.