Is it permissible to kiss the head of a parent if he (or she) only prays sometimes or doesn’t pray at all?

Shaykh Ṣāliḥ ibn Fawzān al-Fawzān:

This is not permissible because (kissing the head) is a type of (showing) love. Kissing him is from love, so it is not permissible to kiss him. However, this does not prevent you from treating him good, (meaning) to treat him good with regards to worldly affairs. As for manifesting love such as kissing the head or other than it, then it is not permissible.