Absolutely! That is part of his identity. And he has a right to know.
Whether to tell him now or later will always be a difficult question; there will never be a definitive "optimal" age to tell him.
Some say before they turn 5 is best as they just accept it as a fact of life.
Many experts consider pre-teen best because the child is developing his own sense of identity and can relate connect to physiological factors.
Waiting until adulthood often creates resentment - after all, who are you to block that knowledge from him?
Nature and nurture both play roles in our lives, but I believe nature trumps nature and therefore, the earlier the better. (Check out the Colorado Adoption Project.) That natural bond between a biological parent and his child will help his sense of self, well-being, and belonging.
It's also very important that you and the biological father agree on your roles. You both need to make it clear always that you both love that child more than anything on earth, and that neither of you ever did or ever would abandon him under any circumstance.
Ultimately, who does not want more love in their lives?
My gut feeling is he is too young but it is a matter of maturity, he should know Yes but it is a matter of when is good , it would be good once he knows and even before if he saw his Bio Dad more, it is well catalogued that children need both parents in their lives. Good Luck