He flies airplanes (simple present). That's the tense you would use for actions that happen frequently. For example, "I teach English" or "I drive to work everyday" or even "I see my grandparents once a year" (How frequent the action is, is not important). "He is flying airplanes" is incorrect because it's in present progressive which is used for actions that are happening at the moment, such as, "I'm typing right now". So, even if you wanted to mean your father is flying an airplane at the moment, you wouldn't say it in plural, because that would imply that he is flying more than one plane at a time.