But, if he plagiarized a section of a book more than 20 years ago AND GOT AWAY WITH IT, you can bet he's done it since. My bet is that if you run all of his articles, especially those published by the more obscure journals, against a data base, you'll find other examples. I'd also bet that he'd try to hedge his perfidy and seek to be able to shift the blame so I'd first check the arcticles where he is co-author.
Similarly, it is much harder for students to cheat this way, as a simple Lexis search can turn up all kinds of things. Glad I went to school back when the Apple IIc was hot.