You must tell him the truth, tradey.
I was brought up by my stepdad (who later adopted me) from the age of 3. I was told right from the start that he wasn't my biological father, but was brought up with my mother lying through her teeth about my bio father.
I recently met up with him, and to be honest for the first time in my life I feel 'complete' in my parentage. I don't wish to take anything away from the man who raised me (he is still dad to me) but I feel I was robbed by not knowing my real father.
I know this is a different situation, but I think your son still needs to know the truth...it was bound to come oout eventually and it's a shame it didn't come from you right from the start :o(