My father does not fast, so is it permissible for me to treat him disrespectfully?

Allah Almighty has commanded kindness to parents and to accompany them with goodness, even if they are polytheists. Disobeying parents is among the major sins. It is obligatory to be kind to your father while advising him and reminding him of the seriousness of abandoning fasting without a legitimate Sharia excuse. One who abandons fasting while denying its obligation is an infidel by the consensus of scholars. If he affirms its obligation, he is in grave danger. You should also frequently supplicate for him for guidance and take useful measures to help him abandon what he is doing.