just be honest with him i would start to just say little things now,dont big it up too much tell him he has 2 daddies one he sees and one he dont,just always be ready for the random questions that he may ask,
I assume your son is adopted by your husband and calls him Daddy? If not, that's what I'd do next, especially the calls-him-Daddy part. Then allmy's suggestion works fine.
This can be handled very naturally. Let it become obvious in family discussions that your married your husband after your son was born. When he is little he won't think anything of it. As he gets older he will ask questions. Answer them.
What I am saying is that you shouldn't present it as a grand revelation.