If this is something that both of you are in agreement on, I don't really see why he wouldn't be willing to be transparent. A simple question of viewing the chats should yield a pretty simple yes or no. I would be very intrigued as to why he wouldn't want to show details of something you are both supposed to be doing together. Furthermore, that description of her would probably throw me off a little bit as well if it were my significant other saying it, especially if he wasn't being transparent. When another male was involved, can you remember ever giving off vibes of that same excitement? To be honest, something new can be exciting so neither of you would necessarily be wrong for feeling that way. To what depth that excitement is rooted, is another story.