Curlywurlyplease I know I a a stuck record but I really agree with CORAL.
You have said " I just know it will change his life forever when he knows that's what makes me sad." But he will not, as Coral say, know the whole thing in one go, he will find out bits and pieces of the story in an age appropriate way and he will deal with these things slowly as he goes along.
"SS haven't offered me a counsellor, it's probably something I will pursue in the future." Ask for help as soon as you think you will need it. Don't wait until you really need it because it may well take time to arrive.
"He hasn't been abused (although suffered who knows what in the womb), it's the birth family that have a horrific disgusting Life."
I think it will be so helpful for you to process all this, whatever horrors there are in the birth family, he will need to deal with this in stages in an age appropriate way. Remember he is biologically related to them so you want to ensure you can pass on this information with out expecting a certain response from him, IMHO because he may not respond exactly how you expect, and you will need to allow him to process this in a suitable way.
"It churns my stomach to have to tell him one day about it all." That does suggest you need help now to process it all to me.
"It's just pretty crap and it hurts." It does, indeed, but try not to worry too much about how you will tell him and when, get some help to process this all and remember that it does not need to have the life altering effect you are predicting. Whatever crimes or vices they have committed, he may find the ability to rise above all this, with time.
Good luck.