So, let me get this straight...
One of your players has made a character that he is proud of and looking forward to play as and the only thing you are focusing on is how to prove him wrong?
To me that sounds like a recipe for disaster.
As a DM you should be focusing on enabling your players, make them feel awesome and making sure they have a good, fun time. I would play as normal and IF there seems to be troubles with the Dream Druid, talk it out and fix the problems together. Don't abuse your DM powers to teach him a lesson for playing the game the way he wants to. Find a middleground where the game satisfies you both.