How do I get him to stop texting me?

I exchanged numbers with an old co-worker of mine. I don't know what his deal is, but he texts me every ten minutes asking me "What's up?" I don't know how to respond without being rude or condescending, so what should I tell him? I don't know if he has a crush on me, or if he's just lonely. He's bordering creepy status. Help?