I have multiple possible fathers, but only one has agreed to test. Can I do a paternity test with just one possible father?

Yes. We can do paternity testing with just one possible father and the child. If he is not the father, you would then encourage the other possible father/s to participate in a paternity test. If the two possible fathers share a close biological relationship (father/son, brothers, uncle/nephew) and only one is testing, then the mother should also contribute her DNA and you must notify the lab ahead of time of the relationship between the two possible fathers so that additional markers can be tested.