Player wants to play character that I don't know how to incorporate into my setting/I'm not comfortable with, and I'm afraid telling him so will upset another player

The interesting thing about this thread is that it is not the player who is being inflexible, it is you as the DM. In fact based on what you've said you haven't even given the player a chance to be flexible. You keep speaking of a jungle but has the player actually confirmed thats where his character is from or just your assumption that all lizardmen come from jungles?

Likewise you say, he became a druid to turn into dinosaurs but there are no dinosaurs here! Well surely lizardmen would have stories like every culture that tell crazy tales? Would it be farfetched for a human child to wish to become a giant?! It seems perfectly reasonable that a young lizardman would hear tales of huge lizards that ruled over the lands bowing to none.

Not to be overly harsh but you have shot down the actual sensible advice because it's not 'believable' (in a game of dnd lul). Here is how I see things, you have built this big shiny world like a train set in your basement, and now you want to show it off but nobody else is allowed to touch or play with it, they must simply marvel at your world building and be in awe at how believable it all is.

/r/DMAcademy Thread