First off, tell him directly that you're not interested in dating him. Don't give him any hope that the two of you being a couple has any chance.
Second off, set up some boundaries. Only reply to his texts on your time. Choose a certain frequency your comfortable with (once a day, once an hour, what have you), and stick to it. Ignore his texts until you've decided you're willing to reply. If he asks why you don't reply as often, just tell him that you're cutting back on texting.