First thing that I would do is separate the father from the other two (you also may want to separate mother and son if the son is over 8 weeks old, as he can impregnate mom and inbreeding chins can have some unpleasant consequences.).
It could be any number of things, including a wound due to injury or attack, a fungal infection, or something else. I'd put some neosporin on it for now, and make sure it's clean until you can get him to a vet for diagnosis.
Best of luck, please keep us updated!