Any nurse?

My dad tested positive for covid he has sense of smell and taste however he suffers from asthma, diabetes and also has had open heart surgery. He has a mild cough however his always had this cough because of his asthma and when he does cough its with phlegm. But my only concern is oxygen levels they go from 85 to 88 to 90.. he says he feels no pressure nor pain or anything on his lungs nor does he find it hard to breath. Only when he does a lot of things and picks up heavy things but it’s been like that even before covid... is that normal? His a 68 year old man so maybe I’m just freaking myself out idk