Personally, I have shot a lot of deer with roundball both .50 but mostly .54 caliber. I never lost a deer with a roundball load. It is all shot placement. The deer you had to track and track I will bet was shot way too far back. The ones you lost, probably shot too far back, high, or just plain missed.
Tell you father (in a nice way mind you) he needs to be more shot selective. Do only high percentage, vital (heart and lung) shots. I have one friend. A nice guy mind you, but he thinks the target on the deer to aim for is the brown. I have told him and told him to pick a vital area, but I think he just see's the brown and fires and hopes he hit a good spot. We have tracked a lot of deer for that guy and normally they are shot too far back.
70 grains should be fine for shots under 100 yards, but if his rifle will handle it, have him kick it up a bit. Even 80 or 85 grains, it will help in penetration and expansion of the ball.
To be honest, I have experience with roundball but none with powerbelts. And I am not trying to start a powerbelt war. But if given the choice between the two, from what I have read, I would probably go with roundball. I know they work.