In the sentence, “I don’t know where he is at,” what is the unnecessary “at” called? I thought it was a dangling participle, but it turns out that is something totally different.
How about “(trailing) redundant preposition”?
There are different terms, depending on how you choose to intepret the sentence.
Quirk and Greenbaum would probably call it a “postposed preposition.”
It could also be termed the particle of a verb (i.e., the verb “to be at,” which is different from the verb “to be”).
It is certainly a nonstandard usage, which make nomenclature that much more difficult.