He probably shouldn't call his group "Yimby Law". Just like if you're not a PE you can't (legally) call your company "Foobar Engineering" in most states.

If he tries to incorprate as "Yimby Law" he may hit a roadblock in some areas. Secretaries of State regulate business entity names and often bar or scrutinize words that imply a regulated profession (e.g., “bank,” “trust,” sometimes “law”) if you are not licensed or not forming the appropriate kind of professional corporation.

However he's free to send a letter, just not incorporate a business called "Yimby Law". He should change it to "Yimby Citizens Group" or "Yimby Institute" or something.