I'm with Niki - my mom & biological dad agreed that when she remarried he would stay out of the picture - I was 6 months at the time she married. He and my other dad were friends as kids (they all grew up together in a small town) and he knew I had a great dad. My "step dad" adopted me.
Because of the arrangement, there was nothing to talk about - no other family visiting, etc - so it was a subject that just didn't get discussed.
Because of that, I felt that my mom wouldn't discuss it - it made asking questions the most painful thing I could imagine as a kid.
And since it wasn't a topic of conversation ever, when my other grandparents sent me a high school graduation gift, my sister was stunned with the news @ 12.
Needless to say, things could have been handled better. My mom and I have discussed this and agree that a continuing conversation would have been best but without new topics (family gatherins, family news, etc) it can be difficult. Not making it a single huge important conversation but part of everyday life in some small way.
My biological dad and his family are now part of my life, too. He also ran out of information to keep up a conversation about me to my brother and sister so my 18 yr sister was a bit shocked with the reunion. My brother remembered the conversations but didn't know where to start.
I guess my advice would be to open the topic. As he goes thru school projects like family trees will make this uncomfortable for him if he has a clue but isn't sure how to approach the subject.
I had the best dad I could ever dream of and I'm sure your sons legal dad is also a fabulous father. Having the conversation won't take that away, but your son may feel he's betraying that love by asking for someone else.
I personally loose "step" and "half" titles unless everyone is involved from personal perspective. Since your husband is his dad, maybe having dad and father might be easier.
I'd be happy to talk to you privately if you think it could help you in any way. Good luck!