I hope to positively add something here being a retired Certified Orthotist and a long range shooter. First of all the location of your father's injury, near the bottom of the cervical vertebra in the neck. 50% of the total rotation of the spine occurs in your cervical vertebrae, thus any little movement can set off a pain issue since these vertebrae, the smallest of the spinal column, feed enervation to a wide amount of your upper body, both muscular and sensory. That's why this issue is a lot more sensitive than can be dealt with here.
Realizing you need to precisely hit a deer at up to 300 yards. I would choose a 6.5 Creedmoor, add a Limbsaver recoil pad, and a muzzlebrake. You may consider reloading and using the 120 grain Hornady GMX, a solid bullet made of guilding metal. You gain three ways here: 1. reduced recoil 2. flatter trajectory 3. better penetration, and no bullet blowup. I am testing these in an old 6.5 Remington magnum mountain rifle of mine and am just amazed at how flat they shoot.
Another thing to consider: the heaviest 30 MM scope you have got. You will gain in early light and late evening vision and also if its big it will almost surely be heavier. Since you are stand hunting you won't be hauling this up and down mountains, so the extra weight will further reduce the recoil. If the visual center of the scope requires a higher comb, so be it, a further reduction in recoil. Thus when you add up all these recommendations this should be helpful to your Father.
Good Luck,
WW