Michael Silverbane
Adventurer
My bullS*** management job is all about dealing with stuff like this. There are a few ways to deal with it that can possibly help:
1. Focus on the problem, not on him. This isn't about him, it's about disruptions in the game and in the fun that you want to stop.
"When we face a big hoard of giants and you start talking about how I'm trying to kill you guys, it begins to ruin the fun for the other players."
2. Talk about how you feel, not the way things ARE. Everything is an opinion so don't state it as fact. The "sometimes we feel like X" routine has a good way of disarming people. "Sometimes I feel like you're not enjoying the game and i worry that this is making it less fun for everyone else". I actually had someone do this to me in an MMO and I realized I WASN'T having fun so I left amicably. Talking about how you feel, how the other players feel, and even how he feels can work better than saying "you're a premadonna."
3. It's too late now but you probably should handle it as soon as you can, as close to the start of the problem as you can. I had a player who was pretty anti 4th edition and I had a talk with him about his ability to enjoy it when we switched. Now he's into it and we're good.
4. Remember to see things from his point of view. Don't discount his own feelings. If he's serious about some of his rants, consider them and discuss ways to address them.
The book "How to Win Friends and Influence People" is full of good tips for dealing with hard conversations. This isn't a D&D problem, this is a people problem that's happened for thousands of years.
This is a lot better than my advice... Which is to punch him in the liver.