I think it's very generous of you and your husband to consider helping this couple out like that...BUT....I cannot imagine giving up all legal and moral rights and responsibilities to this child, yet still have to live next door and see that little one on a regular basis. Especially if it turned out being the little boy that your husband never had.
It would wrench my heart out to be in that situation. Maybe if they lived far away, it might be a different story, but I can't imagine that it would be an easy thing to do in the same neighbourhood. It's easy to be practical and think that you could handle it now, but you never know what havoc your emotions might wreak afterwards.
I don't think I would be able to handle something like that...it would only cause alot of unforeseen problems down the road.