Reference: http://www.alfawzan.af.org.sa/ar/node/2362
Question: Is it permissible to kiss the [fore]head of a parent if he only prays sometimes or not at all?
Response: This is not permissible, because [doing] this is from [the acts of] love. Kissing is from [the acts of] love, so it is not permissible to kiss him. [That said] however, this does not prevent you from being good towards him. As for displays of love such as kissing his [fore]head and the like, then that is not permissible.
