Here's a chart for appx how much yarn you'll need for various projects. For mittens in worsted it gives you 3 different sizes. An adult large says 175. If he has. Hands larger than the measurement then add some. Personally I tend to err on the side of an extra skein. You can always use a bunch of worsted leftovers later for another pair of striped mittens, fingerless mitts, etc.
Eta- I just saw Salmonac posted the link already. Oops!