Here's my take on it -- why is it so sad that your hubby isn't a biological dad? Why is it sad to you that your son is half adopted? Why all the sadness over adoption? Adoption is a great, loving thing and there is no need to be sad about it! If that were the case, then I should be sad that my adopted daughter knows she's adopted and who her birthmom is, etc. But I'm not. I'm all for honesty, and actually it's a little offensive to me, as an adoptive mom, that someone would have so much angst over telling a child the truth -- that he's adopted. It's not a bad word or a bad thing for goodness sake!
I also was adopted by my step dad when I was little. It is a wonderful thing to be loved by someone who adopts you. It is the same kind of love that bio-parents/kids feel. I also have a biological daughter in addition to my adopted daughter, and I can tell you the love I feel for her and my adopted daughter is exactly the same.
Don't be sad about adoption! Embrace it! Let your son know he was loved so much by his step dad that he wanted to make it permanent. (My dad didn't HAVE to be my parent, he WANTED to be, and he was there for me my entire life. We had a special bond because of that. I'm so glad I knew the truth.)
I understand you're torn apart, but you shouldn't be. The truth about our heritage and our life and who we really are is our right as humans.
And it's so much easier (and honest) to tell kids from the start, rather than let them figure it out on their own as some have suggested here. Your son has a great, happy-ending story here! Talk to him about it now, and it will just be a normal story to him. Trust me, it gets harder as they get older to "spring" something like this on them.