I think it must be a matter of body shape and personal taste. For me, the Exos hipbelt is the most comfortable I've ever used on any backpack. It's flimsy instead of stiff, but still padded, so it eliminates any pressure points on my hips and instead wraps exactly to my body shape, evenly distributing pressure.
Thicker, stiffer belts always pinch my hip bones.
But another option might be the Deuter AirContact Lite 65+10. It has the advantage of being fully torso-size adjustable, so you know it will fit him without him trying it first. My roommate bought it after carrying 40 pounds for 4 hours in the store without feeling anything. It wasn't my cup of tea, but it seems like a whole lot of people consider it very comfortable.