What I find interesting on this thread is that this girl and he have been together for nearly 2 years and all anyone can say is "Get a DNA test"? Ummmmm to my knowledge you can't test shit until the baby is born and then the proverbial damage is done.
If my bf of that long questioned my fidelity I would cut his ass off and do whatever I wanted as from that moment on I would consider myself on my own.... which is what ANY WOMAN should do in her position.
I find it very interesting that the woman he has been fucking for nearly 2 years suddenly is not good enough to "be the mother of his child". With all due respect then, why keep her around that long?
I hope for everybody's sake that this is a false alarm and that whether it is or isn't both involved realize that they are most likely not long term relationship material for one another and decide to go their separate ways.
If you don't trust that this girl has been faithfull then you best cut it off now. Be ready for an adversarial custodial relationship AT BEST if she decides to keep what is most likely your baby (if there is one).
I also do not understand how a woman whose man clearly questions her fidelity would WANT to keep his child.