you can lead a horse to water but you can't make it drink

Even favorable circumstances won't force one to do something one doesn't want to, as in We've gotten all the college catalogs but he still hasn't applied--you can lead a horse to water. This metaphoric term dates from the 12th century and was in John Heywood's proverb collection of 1546. It is so well known that it is often shortened, as in the example.