It's not unthinkable to marry someone with the same surname. Or she divorced, kept the tannen name or had the kid out of wedlock and doesn't know who the father is. There are a number of scenarios

Isn't his Grandson referred to as Griff Tannen? He'd be from the male child to have the last name.

Um no. That was not Biff with the girl scout. That was some random guy who knew George. He was probably a coworker or a neighbor, definitely not Biff.

You're right, he had a daughter. In a deleted scene Biff talks George into buying peanut brittle from her Girl Scout troupe, explaining why he had a box of it.
Formerly Eclectic_Flamingo

What makes you think Biff had a son?

If they wanted to show the Tannens as a bully family, in Part 3 Mad dog Tannen was introduced in an old west setting. Why didn't Marty have to tangle also with a Tannen from his generation?
I'm not insane I'm just crazy

I read/heard somewhere long ago that in the earlier drafts of the script that Biff's adult son in 2015 was originally intended to appear, but was cut from the final product for reasons left unexplained.

Maybe it was one of the other people in Needles' truck in part 3

Not Needles, since his full name is Douglas J. Needles, not Tannen. In any case, Biff's son is not relevant to the story so we'll never know.
"You're not thinking 4th dimensionally!"

Verne Brown

Needles is actually a good candidate. IDK who else it could be.
Metallica and Iron Maiden fan who is also a Brony.