Put him in a shower. Like most humans we like to sing in the shower (maybe). But still it's quite enjoyable and maybe talk to him while he's takeing his shower and maybe sing to him and/or whistle to him. *I read this on a parrot training site and heard it from a couple of experienced parrot trainers* Good luck Though!
Here's a video of one singing really good => http://www.cockatiel.com/videos/tequila/