Are his stirrup leathers wrapped and twisted? This method is quite popular here with the Trail Rider’s groups, folks who spend a lot of time in the saddle, often a Western saddle.
I have done this to my Western saddle, but have not had time to go riding in it yet! Stirrups with this done hang to allow leg and foot to slide into position when mounted, no pull from fender trying to be flat again, to affect the knee.
Kind of hard to explain, but the fender strap down from adjustment buckle is wrapped to be round with a leather lace to just above where stirrup is located. Then wrapped part is kind of half twisted to fasten back into the buckle holder. It makes the stirrup hang sideways, so foot is correctly positioned when you put it in the stirrup. I also have a hobble strap just above the stirrup to hold the fender and straps in place.
My local tack store does this for you if you want fenders changed for comfort
My old saddle has the softest fenders, bent to go around your leg over the years. Still have a TINY bit of pull so my knee feels it. With the young horse coming along who doesn’t fit the old saddle, I thought trying the wrap idea was worth trying. Now I just need to get riding and see if this has fixed things for my knee!!