I'm sure he'll love whatever you make him. I think dark, vibrant colors make gray hair pop. How does he look in red? Royal blue or black might also be good choices. My uneducated guess is that if you do a lot of cabling, you might want something other than black. I'm thinking you might not see your hard work unless you were really close to a black sweater--but maybe I've just never seen a black sweater with a lot of texture.
Cotton sweaters tend to not hold their shape really well for me (they get a little "droopy" between washing), so I'd vote for wool.
Just a personal thing, but I loathe bobbles. I'd make sure the person I was knitting for didn't feel an aversion to anything before I put a lot of work into a sweater (assuming I ever actually get the never to commit to something as big as a sweater).