My husbands facial hair doesn't grow on his chin and right under. What can we do to make it grow fully?

He's tried shaving the area over and over, the hair just doesn't come in. People used to make fun of him when he was in his 20's, and call him patches. What can we do?My husbands facial hair doesn't grow on his chin and right under. What can we do to make it grow fully?
He should just keep himself clean-shaven. The situation isn't unusual.