You just can't not hurt his feelings, it will just have to make him sad. Just tell him not to worry because he's incredible & will find a perfect girl soon enough:) good luck! And be extra nice to him for a while!
You don't. Life is too cruel sometimes to work that way. In all honesty, the truth hurts sometimes, and there is no way to avoid that reality. The best thing you can do is just be honest about it, but put it as gently as possible, then it might not hurt so much.