Yoga. Even just once a week. It truly does work wonders. I've even converted my mom and sister, who are both extremely anti-stretching, anti-yoga type stuff. But they both agree that it's great for all that ails them (achy joints, stress, back problems)
If you're looking for a book, I would try something by Jon Cabot-Zin (I'm not sure if I'm spelling this right) on his Mindfulness-Based Stress Reduction. It is a technique that can also be taught by therapists and can include things like meditation and yoga (hatha). There have been studies that show that practice of this really does decrease stress and even helps boost your immune system.
Xanax. Just kidding-that was bad. Umm, running helps to clear my mind. Reading helps. Zoning out in front of the TV. Being social with friends helps. What kind of hobbies does he like? I find my hobbies (shopping, scrapbooking, lifting) help me a lot.