Oh goodness. My heart goes out to your little boy. This will hurt. This may rock his world. I wish you could just leave it and introduce his bio dad as a new special family friend. I guess you probably can't but this will feel devastating to your son. I hope that the man he thinks is dad loves him enough to still be there for him.
Hurting our children, seeing their wounded eyes is the hardest part of being a parent. I know this will be really difficult for you. I wish I had a great answer that would fix everything. I just hope both men can give this boy love he needs. best of luck to you.
Ouch! This is a tender age. First the man that thinks he is daddy must be told. Then, maybe the two of you could approach it together with your son and explain that this daddy was not the one that helped create him. Hopefully this guy that thinks he is the boys dad will be on board, if not, then that is gonna complicate things abit. In that case you might have to go it alone. I would introduce the boys father into his life and let them develop a relationship and see how it goes and if your child develops a strong bond, it will make the transition much easier. Otherwise, it could develop major trust issues from your son to you. But whichever way you go, he has a right to know the truth. I feel for you going thru this it is gonna be hard.