What to do about this guy?

OK you just said that this guy was not your friend, so who is he to you and why should you or why do have concern in the first place? You just dragged his reputation through the dirt and belittled him about how much of a negative person he is. Secondly if he is blunt like you say he is, be straight up honest with him and just tell him what you think. But don't be a jerk about it. Be tactful about how you address an issue. Sounds like you don't really care about him but simply you pity him.