When do I tell my now 3 year old that the man he calls dad is not his biological father?

I have been with my husband since I was 6 months pregnant.  We married after my son was a year old.  He is now 3 years old.  That is the only dad he has ever known.  We recently had had another baby boy together. The biological father has never been around and never will.  When and how do I tell him the truth?

[allmymarbles]

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.

[AnnieBrooke]

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.

[cheshchesh819]

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,