He's sober now. Many years ago, or so it's said, he really cleaned up his lifestyle.
How long ago did you see him? Because I also saw him in Camelot, many many years ago, and he was obviously ill (he didn't go on the next day), supposedly from hypoglycemia, but who knows? However, although he could barely stand up, he did an excellent job. The man really can act.
According to what I've read, he's been signed, so lets hope rumor is right and he's clean and sober. If he is, I think he'll do pretty well as Dumbledore. He's got that bit of mischief in him.