A sleep test if you have the resources (health insurance and such). Most common cause for apnea is obesity but there are other potential factors like issues with tonsils, sinuses, septum, turbinates, and/or adenoids that wouldn’t require a cpap but other things like surgical correction

Additionally lifestyle changes can make a huge impact especially if it is related to obesity

Nowadays you often can do sleep tests for things like apneas at home, you usually dont have to go to sleep centers. It can be worth it to make your dad aware of this. Ive had older clients who were very avoidant of sleep studies before they found this out because before a few years ago it was far more common that sleep studies were a much bigger pain in the ass. Youd have to go to the sleep center and sleep there, hooked up to a bunch of machines, uncomfortable bed, not necessarily on your sleep schedule. Now thats really just reserved for certain sleep issues like narcolepsy and severe insomnia

if you can’t get a sleep test and cpap the old school way to manage apneas was to sew a pocket to the back of a tshirt that held a tennis ball, which would force you to sleep on your side. Not ideal but better than dealing with the health impact of an apnea. Not to inspire fear but apneas are terrible for your health. They cause you to wake up briefly and return to sleep.

This happens fast enough for you to not remember and as a result the “cycle” of sleep is interrupted. If the apnea is severe this can happen many times per hour or even per minute, causing you to never get restorative sleep. You “sleep” all night but feel exhausted all the time because you never enter the deeper cycles. Luckily it’s not an immediate danger at all but after years or decades the effects compound just like having a consistent extreme lack of sleep would