Its very tricky sammi as when you have your own you prob won't love him as if he were your own, just cause he isn't.Not being mean but its such an emotional thing having your own child. I even found with my kids that you feel rush of protectiveness towards your newborn that even extends to protecting them from their older siblings IYKWIM?
Is there anyway he can be with DH's parents or someone until the time comes for him to be meeting his new sibling so that mum isn't involved? I wouldn't want my DH's X being one of the 1st people to see my new baby either!!!!
Sorry I haven't been more help, i will think about it some more LOL