I've worked with two Yugo M70-based AKs in the past two months.
The two models in question were from Century, and were PAP versions set up for optic mounts.
Now, this is a Yugo-specific issue, so it needs to be approached from that literal angle & with some specific Yugo analysis without making general AK statements or recommendations to switch to an AR.
On both, the US-made WOOD stocks are angled higher to provide a higher cheekweld WITH optics mounted.
What that does is force the cheek down lower onto the stock when using iron sights. I ran into the same exact problem, it forced me to rest my cheekbone ridge directly against wood on that stock, and even with the low recoil of the 7.62x39 round each shot was painful. Had a visible bruise the next day after shooting the first one, shot the second one with an optic mounted & had no problem.
Since I've never tried a non-optic Yugo AK version, I don't know if all of the Zastava-made M70 derivatives use that higher-angled comb, or if it's just those built for optics.
Does your Dad's gun have either a rail on top of the cover or a side-mount rail on the left side of the receiver?
If so, that's your problem right there.
If not, I'd say it indicates all or most of the Yugos do have the higher stock angle, and if that's the case, that still remains the problem.
Stock changes may or may not help.
Selling the Yugo & buying a standard AK with a standard stock drop would solve the problem.
I can sympathize with your Dad, I flat would not shoot one of these high-stock guns using only the iron sights.
How long has he had the gun & did it come from Century?
Denis