He looks kind of stuck there with the rock wall next to him. Somehow cut off from his castle behind him. His throne seems more massive than the other kings, more plush too, along with his crown and robes. I tend to think he is more workaholic than alcoholic. I get the impression of the father who has spent late nights at the office or weekends on the road to advance his career and provide all the best (homes, cars, etc.) for his family, and eventually realizes that he has missed out on being a part of his family--he was never home.
But there is that shadow of grapes on his throne too. Like it's there hanging over him. I can see where addiction could come from his pain, perhaps his loneliness led him to drink. Even if he has learned to control his addiction, the shadow is still there, the possibility that he could fall off the wagon at any time. His sadness comes from knowing that even in times of happiness and comfort when everything seems alright, it can never be taken for granted. Which brings us back to world weariness, I believe.