"Nichols said that Bill had admitted that he "shoots blanks" after contracting measles as a kid rendered him sterile, and added that Hubbell was the real father of his daughter, who is currently pregnant with her first child with her husband Marc Mezvinsky."
If True then Danny could not be his son. However, I think that Danny probably is his son and Bill was giving Nichols a BS story about being sterile as a cover story since it was sort of obvious Chelsea was not his.
(We are getting down to some of the earliest levels of FR / Clinton Whitewater history here!)