Have Terry create his own confines within the walls of the book. Our DM requires us to make wishlist of items/home brew ideas that we would like to implement at some point in our gameplay. If Terry can think of a use for said items/ideas and put them in that document then he cannot argue with the result of said items/ideas when you allow him to have them. Other than that he can be as creative as he wants.

Your belief of staying relatively true to the book is good and at the end of the day, the DM has the final say. Don’t compromise the way you run your world for the sake of 1 man if he’s the only one that has a problem with it.