Among the bonus features in the first Back to the Future DVD set is a list of frequently asked questions, with one being "if Griff is Biff's grandson, why don't we meet his son?", with the response "since we never learn Griff's last name, how do you know Griff's mother wasn't Biff's daughter?" And there's two things about that:
1. Fine, he might have a daughter, but why don't we meet his daughter then?
2. Even if Griff's last name was Tannen, Biff could have still had a daughter but no son, if Griff was born out of wedlock.
Of course, drafts of the script say that Cafe '80s is owned by Biff Tannen, Jr., which means that he does have a son (though that doesn't mean his son is his grandson's father, he could have had multiple kids), which is pointed out in some of the fact tracks on the set. And of course Biff had a son on the animated series, but it's debatable whether that counts as continuity.