How to use iMessage between iPod touch 4 and iPhone 3GS?

I have an iPod touch 4 and my brother has a iPhone 3GS and when he try's to message me via my email address it just sends me a normal email instead of coming up in iMessage. When I try to send something to him the send button doesn't work?

How can I fix this?

He may have iMessage turned off. Tell him to go to settings-messages- and turn on iMessage. Then it should work. If it doesn't, it maybe that it just sent as a text message (which iPods can't do) or its a software problem that you should consult with apple about.