A: I agree in being honest, too. Secret often backfire into a worse problem. The sooner you explain things to him, the better; the longer he takes to find out, the more betrayed he may feel. Tell your boyfriend now that he is not the biological father, but emphasize that you love him and that you want to be a committed family. However, the fact that he is not the baby's father may upset him, so be patient while he adjusts to the information.
I noticed that this is your second letter at the same time, asking for some input about family issues. You sound like someone with good judgment who wants to make things better. If there are a lot of challenges at home, consider talking them over with someone who can help sort them out--a friend, a clergy member, or a certified counselor. Perhaps the family doctor can also suggest someone.