How should we go about this? Although my husband and son have a wonderful relationship, my husband is quite concerned that our son will no longer think of him in the same way once we tell him, or even worse, he will want to see his biological father.
-- Truthful Parents --
It would be good for both of you to tell him, to assure him that his genetic father never even saw him so he was never rejected or unloved. Explain that his adoptive father is as bonded to him as if he had participated in his birth.
As difficult as it may seem to you, your son will probably accept it. He may wonder why you didn't tell him before, and you can explain that you didn't think he was old enough to understand earlier. If he does ask to meet his real father, and he may not at this age, explain that can happen when he becomes an adult, and that it will be his decision.
There is little likelihood that the new information will hurt the relationship, but if your husband needs some reassurance of that, remind him the fact that he actually chose to adopt your son after he already knew him is huge proof of his love and commitment to the child.