I know you are scared.
I am here for you. I'll be with you every step of the way.
There are a few things though, I wonder if you realize. Nobody loses custody of their other children, or their child, even if the baby does have FAS. All these horrible things that you are imagining: we call that "catastrophizing"; making things CATASTROPHIC. Granted, this will be difficult, but I really don't think that it will be as bad as you think.
If you didn't know you were pregnant, then your husband didn't know either -- you certainly wouldn't have done this on purpose, and he will realize that.
Your daughter know you as a loving Mom; you seem hard on youself, but I doubt you teach your girls to be judgemental -- so they won't judge you. Especially if you are a good Mom to your son. What is his name?
I don't think your husband will kick you out either... be armed with information so that all of you can seek support together.
If you feel you can't say it out loud, write it down, ask to speak with the doctor, and hand him the note. He has to know -- it's the only way you can know for sure. And if that is the case -- is your son any less of a lovable baby? Of course not. He needs you. And you need him -- and your husband and the girls.... so stop beating yourself up.
Your guilt is becoming all consuming. You will find relief when it's all out in the open and you are all working toward the same goal -- taking care of that little boy. He can have a perfectly normal IQ.
The faster you have a diagnosis, the better off all of you will be.
Earlier, I asked you if you could have post-partum depression. Is that possible?