First, you need to decide if you can stop cheating on your husband. If you can't honestly say you want to be faithful to him for the rest of your life, then now might be a good time to re-assess just what you want from your marriage.
If you want to stay with the husband, and be faithful to him, you next have to cease all contact with the boyfriend. He's only going to be a major temptation.
Will your husband be happy about the baby? If so, I wouldn't tell him that he might not be the father. If your husband will be happy being a father, and helping to provide a stable, loving home for the kid, then I say don't tell him anything. I don't think that honesty is always the best policy, when honesty is only going to create pain, hurt feelings, and distrust.