Keep his own operation as is and help dad out as much as he can with whatever his dad needs as he has time. See if the relationship has improved. Then go from there depending on how things work out.

If they seperated operations under less than ideal situation................ well sometimes people and attitudes don't change. On the other hand, sometimes they can if the personal incentive and attitude is there.

Trust but verify by moving forward at a pace that can easily be reversed.

The relationship may be more important to cultivate (or no-till if that is your thing) than any acreage involved.