I would suggest before doing anything have you dad check with the DR. My buddy had open heart surgery a few years back.I realize he's 15 years younger. He's hunting with a Ruger M-77 in 7mm Rem mag. Your dad's rifle is awful light that's where the recoil is coming from. My wife hunts with a Ruger M77 Compact in 7mm08. Very little recoil.
I wish your dad the best with medical issues. Hopefully you'll find the rifle and caliber that he can use.