Can i get custady of a child that i raised with my name but was proven wasn't mine after three years?

My wife cheated on me three years ago with a convicted child molestor. She got pregnant and had his baby. For three years i thought that i was the one that had gotten her pregnant. But with a reacent seperation we had a blood test that showed he wasn't mine. Well he is mine no matter what. I am trying for custady of the children. I have four with her. Now sense we found out the youngest wasn't by me she is useing him so i cant get the children. Was told i wouldn't have any right to him even though his real dad is a convited child molestor. Sense i have no right to him that i wouldn't be ale to get the other children cause they wont seperate them. Well i dont want to seperate the children. I want custady of all four of them. Even though the youngest might have another biological father. I will always be his real father. I think i should have rights to keep him as my own sense i raised him sense he was born. Is their anyway i can go to keep him as my son. Please Help.