Yeah, there are many systems out there designed so people can do super hero stuff and have super hero stuff and the game doesn't rip apart.
D&D isn't one of them. It may have all the cool trappings, but mechanically it doesn't support some levels of fun. Especially when it centers around one character as opposed to being part of each character. (My race is better than your race.) When half your party expects to be playing half competent swordsmen who are one step removed from peasants, and the other half come in wanting to play demons with intelligent weapons right out of the box because it's part of their favorite anime....expect friction, disappointment from everybody and a lot of whining about "but why can't I?" Lots of players are only into RPing for their own experience, not realizing that getting to dictate everything about themselves isn't what the fun is about.
D&D is notoriously bad about having some party members get ridiculous strong, while others basically stay totally ineffective. It got so bad in one game, because all the older players had relationships with gods and all these granted powers, that all 8 people in the party eventually took deities just so we could feel powerful too. We ended up calling it the God Party, because our whole time was consumed with pleasing our gods and doing errands so we could get another bit of power.
FWIW, I've tried multiple times to start high level parties where people either had lots of creative control, or lots of random goodies to start out with. It's NEVER worked well. EVER. The first time you throw out a monster and a player is hittin in the +7 to +10 damage range, you're going to hate yourself. The first time a wizard uses 4th level spells to solve what was supposed to be a roleplaying problem, you're going to hate yourself. It is far, far less enjoyable for everyone when they get the things they want to start with, and get the tools to trivialize situations before they've gotten into them.
I'm not kidding when I say that keeping high level parties from rolling over you as the DM takes experience, practice and a lot of failure before you find the balance. It sounds like this player is pushing you in the direction where you're going to lose control of him and the situation quickly.