ALso SM - you can't use your DS's almost preconditioned 'love' for his father as an excuse not to do something!
Love is twisted and tortured at times - most of us in abusive relationships know that. It isn't fair that your DS thinks that what he feels for your DH is love! It isn't - it isn't at all!
Please give him the chance to know what real love for a father figure is - to have someone cherish and mentor him into adult hood. Your DS is a victim of child abuse - and it is not fair of you to allow that to continue!
I know that sounds harsh but it isn't any less true for all that!