He's not the rancher and doesn't own the cows, So can't call him a Rancher.
He's no longer a boy, so Cowboy doesn't fit.
His profession 12 months a year is a hired hand to take care of cattle on open ranges. So cow handler it is.
A funny story. This old guy rode into the trail head one afternoon. We were all standing around looking at a strange trailer that was unloading a couple of camels. Where these folks got camels, Who knows but they were out for a ride. When I saw them getting unloaded I took my horses over for a learning session. It never hurts to expose your horse to strange looking/smelling creatures.
Any way this old cow handler came by and saw those camels and said. "Boy am I glad I saw them. I would have notices those strange hoof prints on the trail and been wondering what creature was stalking the hills all summer"