Sorry you're going through this.
Read your update post and I do want to give you a lot of support.
It sounds like your dad was harsh and that must have hurt, but it also sounds like he truly loves your DS and you and wants the best. I'm sure it will turn out really well. Your DS can be the center of attention for a while, still get the help he needs, and you can focus on your other LO and yourself and get yourself on track towards being what you want to be in every area of your life.
It takes a lot of courage to recognize and do the best thing for your child, even if it is breaking your heart, which I imagine it is. It is the essence of AP, though, to meet your child's needs at every stage and it sounds like your eldest son's needs, at this stage, are going to be better met with your dad and stepmom.
If you haven't already (and I know you didn't want "shoulds" in here!), I would sit down and explain that you're *not* abandoning him and that this *isn't* about him being "bad" but more about you all getting on track and him getting a lot of special time and attention from grandpa.
Hang in there. You sound strong and like you're really trying to create a good family despite all your difficulties in the past.