How to switch between text message and iMessage without turning 3G/iMesage off?

Okay so my boyfriend just got an iPhone, but he doesn't have much data, so when he's out he turns off his 3G and switches to texting. However when I try to text him, it still says I'm sending it as an iMessage. It won't send him texts. The only way I can send him a text is by turning my own iMessage off. But I don't want to be turning my iMessage off because that is the only way I talk to everyone else in my phone book.
Is there any way I can send him texts while he's out and then switch to be able send him iMessages when he has Internet without turning my iMesage on/off constantly and being unable to contact anyone else while it is off?

It should switch automatically when you hit send. If that's not working i don't know what's up

If it doesn't automatically change you should be able to double click the imessage once you've sent it and click "send as a text message"