"Departed" Problem Child? Are you finally free?
Well, back in 2e there was a Dragon Rider kit (or some such) that guaranteed a character had a Dragon mount.
So I gave him a Mist Dragon.
From the Monstrous Manual:
"Mist dragons are solitary and philosophical. Their favorite activity is sitting quietly and thinking. They hate being disturbed and dislike conversation."
"Combat: Mist dragons try to avoid encounters by assuming mist form. In combat, they quickly use their breath weapons, then assume mist form and hide in the vapor - where they launch a spell assault."
In RP, I had this Dragon be annoyed any time he was addressed, let alone being asked to help fight. Most often, it just hung out in Mist form and generally kept to its own.
Hilariously, this worked for the player as he was a Lizardman; he had to regularly moisten his skin or start suffering Con penalties, so this a win-win in everybody's book.
Just because he wants a Dragon, doesn't mean the Dragon has to LIKE him... just obey him.
Also, he shouldn't be able to "build" any kind of a dragon - he should get a stock Wyvern with all of the rage and horse-likeness it entails.
Also also (from the same Monstrous Manual):
"An adult wyvern consumes the equivalent of a man-sized to large creature once per day. This could translate to a horse, pig, or a handful of sheep. While it eats them whole, the bones are not digested, and neither are metal objects. The wyvern eats carrion only if desperate."
Have fun with THAT.