He needs (like celtic-_-pride and footey said before) to improve his ball handling skills, both hands but mostly left hand.
I couldn't care less about 'swagger' or 'alpha-guy' labels, this is a sport and you need to take advantage of your skills to win the games. Confidence is a thing that comes from mental approach, but also from experience. The more success you get, the more confidence.
Jeff is a very gifted player, and he's been playing very different roles in his career, but he's never been given the ball (in the pros, at least), and sadly, he's never developed his ball handling. That would help him not only in iso jumpers, but also to attack the basket. He makes it look outrageously easy sometimes, but he needs to work on his handles to be even better.
There are guys like Jordan "better than Michael" Crawford who has crazy confidence, almost never based on success, and that doesn't make him a better player. Of course is funny to watch Steez do his thing, but if you want to win games, team game is key. Another example of misunderstood confidence is Big Baby, ya know...
I wouldn't want Jeff to turn into a chucker like Kobe, hero ball is disgusting and the example of everything that's wrong in our world (hehehe). Really, that's anti-Celtic stuff, and as much as I love the Captain, I hated those iso plays even when they resulted in great shots. I love clutch shots, but they're better when they come from the coach's playbook, a great pass by a teammate... but that's me.
There is only one Paul Pierce, and there's only one Jeff Green, and you should appreciate who they are, and stop questioning what they won't be.