Really clingy guy won't stop messaging me, what should I do?

[Pincat]

So there's this guy I knew briefly about three years ago. He's 17, a year older than me. Lately he finally got a Facebook account and added me. Then came the torrent of messages before I messaged him back, and when I finally did he immediately told me his tragic life's story and starting going on about how no one loved him. I decided to go with it because he was obviously extremely lonely, and from there he just got clingier and clinger and expected me to text him back immediately and freaked out if I didn't, texting over and over again until I finally told him it was too much and I needed him to cool it. He was fine with it and it was better, but now he's back at it, threatening that if I don't message him back he'll kill himself and calling me over and over again. I don't know what to do - I've told him the same encouraging words practically every time he texts me, but he just demands my constant attention and I really don't want to give it anymore. Any advice would be wonderful.