I know a lot of men that are still in very good shape at 50, but they have to work for it with many hours in the gym or out in the mountains. And of course, a healthy diet appropriate for the age is a necessity too. Being in a death metal band can be a very physically demanding activity. He will need a lot of endurance to keep up with all the young, buff kids on stage these days. Of course, the performance could become part of his exercise routine too. Just remember to stay hydrated, especially if he's going to be out and about this year with the heat waves and all. Sweat is principally water, after all. I'm sure he'll have no problems. I can just see him right now, moving around on stage with his muscles rippling under beads of glistening man nectar, his bare greying chest hair now a coat of arms that will embolden hundreds more of his kind. He'll be just fine.