What a story! You have been through so much. I would wait until you son is 18 years old, or until he graduates high school. There is nothing worse, as a kid, growing up knowing that your biological father didn't want you and relinquished his rights. If you talk to your family and your husband's family and let them know-"we are going to wait until he is older to fully understand what happened", I know they will honor your request, and not speak of this so it doesn't "leak" out amongst the family. I think trying to tell a child (I have a 12 1/2 year old) is alot. It's one thing if the father died, but trying to tell your son "your biological father is a jerk, he wanted nothing to do with us, but your dad that raised you stepped up to the plate and is a far better person, and wanted to be your daddy". That is alot for a child to process. Like I said, I have a 12 1/2 year old, and I can't imagine having to tell him something like that. He is a great kid, he does have some self esteem issues (most kids this age do), we encourage him and praise him alot-but it's just hard being a kid now a days.
My brother and his wife are in a similar situation. She has 2 boys from a previous relationship-thier father was killed in a car accident 5-6 years ago. She got into a bad relationship, broke that off. My brother started seeing her again (they dated in high school, and remained good friends for 10 years) she was 4 months pregnant when they started dating again, the baby, not biologicaly his. They got married when the baby was 2-3 months old. That baby girl may not be biologically his, but don't try to tell him any different. He says the same thing "I'm her daddy, and that's all that she needs to know". We all know he is not her biological father-both sides of the family. We are all keeping this a secret until it's time for her parents to tell her. Until that day comes, we will love the mother (we have become close friends), and we will love the baby (she is now 1 years old) as if she were our own.
I have always believe love really has no boundries. We just open our hearts to any and all situations, as long as the intentions are good.
Good Luck!!