It sounds to me like he is already into it. Unless he is a professional historian specializing in the medieval judicial system, there is no reason for him to talk about it a lot
Maybe it is worth it to take the risk of opening your mind to him. After all if he really is your friend he will tell you that you are very sick puppy but will still be your friend.
Remember that it is OK to be a pervert if you are a happy pervert.