Well, sonograms done in the first trimester are pretty accurate. Ones done later in the pregnancy are not as accurate. Using the 8 weeks sonogram, count back and see if your son was with her at that time. I got February 19, but the ultrasound is not exact and could be off by a few days. You may have to wait and take a DNA test.
My family and I have been in your place before. When my brother's ex girlfriend became pregnant he refused to listen to reason and claimed the child without having a DNA test done, even though he knew she had selpt with other men. (He got her pregnant once before and demanded a DNA test; she lucked out that time and the baby was actually his. After that he felt that he owed her the benefit of the doubt the second time around.) My nephew is now twelve years old. I love him and will be his aunt no matter what but to this day I don't know if he is my biological nephew.
So, it doesn't have to be a terible thing. I wouldn't trade Chuckie for anyone in the world. I just feel bad because the mother has possibly cheated the real father and her son out of the opportunity to know about one another.