What can a son offer for his deceased father who sometimes used to miss prayers, and is it permissible to perform make-up prayers on his behalf for what he missed?

It was previously explained that occasionally missing prayers does not render one a disbeliever according to Imam Ahmad and Ibn Taymiyyah. It is recommended to frequently supplicate, seek forgiveness, and give charity on behalf of the father.

As for making up the missed prayers, it is not legislated, because prayer is an act of worship that does not accept proxy.