Although there is a very small chance that the other guy is the father it is time to really focus on your baby and yourself, and be practical. Getting stressed is not good for you or your baby. There are no prenatal tests that can determine who the father is during pregnancy.
Try not to dwell on it too much during your pregnancy. This is issue is temporarily and will be sorted out as soon as the baby is born.
It's bound to be tough going through your pregnancy without sharing it with somebody you know is your baby's dad. Talk to family or your parents. Try to gain emotional support during this tough time.
Once your baby is born, you may be so focused on raising the child that your worries will slowly diminish.
The one way you'll get a definite answer about the dad is a paternity test
A paternity test is simple and quick. You, your baby and the assumed dad will each have to give samples of cells taken from inside your cheeks. The cells are checked to see whether the assumed dad shares any genes, or DNA, with your baby. If he does, that means he's the real dad.