To be honest, guys can be dumb sometimes so you have to throw the answer right in their face (I know from many experiences :P). But, you just have to be straight forward with him. Tell him that he had his chance and he just didn't cease it. Tell him sorry and wish him luck in his future relationships. Honesty can hurt, but it's the only way to get the message across.

I would ignore his messages, he doesn't seem worth your time or energy... to be perfectly honest. Replying to his messages could start a whole new dialogue which might get you both worked up into an unnecessary argument. If you still care for his feelings (which you really shouldn't, but that's life) respond and tell him that you've moved on and that you hope he will too.

Definitely ignore him! He didn't care to love you enough then, and now he misses you? Not to sound like a total jerk, but he's just looking for booty. You have someone in your life now, don't miss that chance.