Legality of using factual information/people in fiction?

Tweaking won't solve the problem. If it is clearly his work it'll still be clearly his. IF in your fiction he does not function as an active character doing things that he in real life might find offensive or objectional then stating aspects of the theory and attributing them to him is no different than someone writing a critical or academic work citing - AND CREDITING - his research. The key is that you may not appropriate HIM and represent HIM as actively participating in your story. He is not a public personage and is therefore not fair game, unlike clearly fictionalizing a conversation with Tom Cruise or Laura Bush for example. If you attribute words or actions to him you would need his permission. If you simply state the fact of his work and his ownership of it there is no harm.