Thank you for your response. Just to clarify. My grandfather is one of 6 children, him being the first. We know the identity of his mother, but there has always been a possibility that his father is different from the other 5 children. What I'd like to do is confirm whether his father is actually his father, or not. Does that make sense?
I have cousins that are descendents of those other 5 children, a couple of them would be willing to test.
I was also thinking another way to look at this is: I have cousins that are descendents of my Great-Great-Grandfather. If the disconnect is my great grandfather, than really we're not cousins. So a DNA test would say if we're related or not which would tell me whether my Great Grandfather is the disconnect that we think he might be.