While I personally haven't had to do this, both my mother, sister and brother did. (Wow, sounds like I've got a crazy family!)
My sister got pregnant in high school and the father didn't care, but she had another male-friend who wanted to marry her and be the baby's father, which is what they did. They since divorced but that man has been her son's father in every since of the word, he's even listed as the father on the birth certificate and the biological father signed away his parental rights. When her son became seriously ill around the age of 7 and needed a kidney transplant, they (my sister and her ex) decided it was best to tell their son the truth. He was upset, but they were very clear in explaining his dad has been and will always be his dad and it didn't really matter that his genes came from another man. Then my sister had to call the biological father and ask if he'd be willing to be tested to donate a kidney. He agreed, but wasn't a match. He asked if he could meet my sister's son and she said that was her son's choice. Her son said that he had no interest in meeting his biological father ever and never has, and now he's in his twenties. He is very close to his father and has never had a reason to doubt their relationship and has never had an urge to meet his biological father. Anyway, it was hard on my nephew, but he has always been very smart so he understood that it's not genes that bind you, it's love.
As for my mother, she had an affair and became pregnant with my brother. But my parents chose to raise him as their own. (My parents divorced while I was in high school.) It wasn't until my brother was in his twenties that he (and the rest of us) found out that our dad wasn't his biological father. It was devastating to him and the rest of us. It ruined the relationship between my brother and father because my mom made the decision on her own to tell him even though my father asked her not to and my brother just couldn't handle it. In hindsight, my mother said it was something they should've told him when he was younger, so he could grow up knowing that his dad loved him no matter what. My brother doesn't have a relationship with his other half-siblings (from his biological father) and he did meet with him a few times, but said it was just odd and uncomfortable (his biological father passed away a few years ago).
My brother met a girl who had a 9-month old son and was not married. They fell in love and got married and had a little girl. Eventually, they divorced, but my brother was the only man her son had known as a dad and they wanted to keep it that way. Although they were always open with him that my brother was not his biological dad. Over the years, my brother would take both kids when he had visitations with his daughter and never treated him differently. Then one day his ex called saying she wanted my brother to legally adopt her son because if something ever happened to her, he would technically go to his biological father who he'd never known. The biological father agreed to give up custody, my brother finally adopted his son (with an agreement that he would never legally have to pay child support even though he did support him) and his son was so happy. They have a great relationship.
Anyway, from both my mother, sister, and brother they all agree that knowing when you are younger is better. It's a hard secret to keep and it will just eat away at you. And chances are, he'll find out sooner or later and it really should be you and your husband who tell him, you certainly don't want to him to hear it from someone else because he really might feel betrayed for being lied to all these years (like my brother). I think the most important thing is to let him know that you have always been there for him, that you'll always be there for him. That you are his mother in every since of the word, just not biologically. You might also want to consider adopting him so that legally, he is yours.
Good luck with whatever you decide to do! And congrats on the pregnancy.