Most commonly if it's not aggressive, they do it to their humans because they are feeling horny. However if he is also spending a lot of time grooming himself down there it might be time for a hair ring check. If it's just him feeling horny some people have had luck giving their chins a chinnie buddy (fleece stuffed fleece stuffed animal) to take their urges out on. You can also take your hand away and say no when he does it.