Steve made the switch to an iPhone from his old Palm Pilot. The interface is very similar to the old Palm, but the iMessage app sometimes defaults to text message instead, charging him. Leo says that iMessage has never really worked right. There are internet only text messaging apps like Google Hangouts that he could use instead. There's also What's App, which also uses data. The addressee has to have a similar setup, though.
It's also possible that the addressee may have also disabled iMessage on their phone. He'll also need to be sure his "Send as SMS" option is unchecked in settings.