I have finally built up the courage to text this guy and I don't know what to say should I say hey what's up or something else? I can't engage in conversation because he doesn't have a texting plan so what do I say (keeping it short and simple)

Just text him when he replys back continue to text for about 2more text messages then tell him to call or tell him that you are about to call him because you know that his text messaging isn't free and you have something to talk with him about but ask if its all right for you to call call him or him to call you however it works out!! I HOPE I'VE HELPED YOU OUT

I would start with something simple but that makes him want to talk to you more because that's the idea. I wouldn't end with a question siply text him something and if he responds with out being prompt to then you know he is interested in talking to you. Something like...

Easy Going Simple...
"Hey (his name here) just wanted to say hi and see what you were up to give me a call when you can"

Maybe you're a little on the goffy side or if he has a good sense of humor...
"Howdy partner! lol Just texting to see whats up text me back when you can."

Very Simple...
"Hey it's (your name here) just seeing what's going on..."

pretend his name is josh =p
be lyk:
"JOSH!!!!!!! i finally got ur number =p ttyl on msn hopefully.. bye!"

Just tell him hey. If you want to, I would call him over text him, especially if he doesn't have texting in his phone plan and can't text back, it's always best to have a two sided conversation.