Interesting question! I think we can help. But first:
1. Should this just be a simple one-time question as part of a larger scavenger hunt or quiz? Or do you want to set up a small project related to it (something like "find trees for each of the following languages")?
2. Is this meant only for your brother who knows about sociolinguistics, or should it be an accessible question to anyone without any specialization?
One thing that comes to mind is a sort of dialect mapping of regional vocabulary, and maybe the answer/clue for the next step is one of the words, but first the participants would need to take the words and set them up on a "map" of where they belong in the country? Assuming this is American English, you'll find various "trivia" lists of regional words online. Here are some good examples:
https://www.businessinsider.com/american-english-dialects-maps-2018-1 or
https://www.rd.com/culture/regional-sayings-phrases-words/ or
https://www.buzzfeed.com/skarlan/the-ultimate-regional-vocabulary-throwdown -- and then of course from that you can guess which ones you'd know yourself to make sure it's not too hard. (Y'all, pop, hoagies are some good examples, maybe also bubbler and lightning bug, and you could find a few more known terms for specific cities like
https://www.huffpost.com/entry/words-new-yorkers-say_n_4179188 -- maybe "subway" for New York, or "the shore" [instead of "beach"] for New Jersey, etc. Maybe cliche examples like "aloha" for Hawaii or "dude" for California, even if that's not really representative.) Then have the searchers try to simply match up those given words to regions on a "map", like labeled trees or something.
That does require some knowledge about dialects, but it isn't too hard to at least follow along for non-specialists. If you do want something more technical, let us know.
(If you want to get a bit nerdy in the directions you could include the word "shibboleth" which is a term specifically referring to the way someone pronounces a word, or a vocabulary choice. You could say something like "then find the shibboleth for [category] which will lead you to the next clue".)
