The child needs to go to counseling. He's old enough to start to work out a plan of what he wants. You need to think about what YOU really want too.
So far, you mention him as "step child" or "child" and talk as though his connection to you is through AH and temporary custody. Taking custody of a child for life is clearly no simple issue. As you have kids already of your own I'm sure you know.
If yoiu are thinking about "Keeping" (it makes him sound like a puppy) your stepson, then you must examine the reasons why. Is it out of guilt? Or fear? Or do you truly want to be a mother to this boy, forever, because you love him like he is your own child. As you are concerned about keeping AH in your life through the child, you may want to look at that. The kid isn't a pawn or a conductor through which adult stuff should pass.
IMO, AH will always be in your life anyhow...at least in your mind. But I think if you really wanted to keep custody of his son you would already know.
hugs....