You Can Lead a Horse to Water, but You Can’t Make It Drink

What does the idiom “You Can Lead a Horse to Water, but You Can’t Make It (Him) Drink” mean?

Definition:

It’s very hard to force someone to do something against his or her will.

For example:

“I know it would be good for my son to go to the state university, but he doesn’t want to go. You can lead a horse to water, but you can’t make him drink.”