Hoew do I let him down without tearing him apart

Dear Ask Bridget,

I have a problem.I met this really sweet guy a couple of weeks ago. We talked on the phone a couple of times and it was cool. But now he calls me every hour on the hour and if I don't answer he'll leave a 5 minute message. I want to let him down and that I just want to be casual friends but I don't want to hurt his already delicate feelings.Got any advice.\
Thank you,
Jennifer