I know of no place in the Vinaya that states a bhikkhu cannot disrobe.

If he no longer has any interest in the bhikkhu-life, the tendency will be for him to become lax and a bad example for others.

His Dharma friends therefore will try to re-fire his enthusiasm.

However, if that is not possible, becoming a good layman may be better than being a bad monk.

Nevertheless, in some countries there is a cultural expectation of “ordaining for life” and a corresponding stigma attached to disrobing.

There is a tradition (but not a rule) about a bhikkhu not re-ordaining more than seven times.

Learn more about the Theravadin Buddhist Monk’s Rules here:

The Bhikkhus’ Rules – Guide for Laypeople