There is a significant difference between a person who requires a bit of assistance here and there on mobility issues and someone who requires assistance most of the time and especially in case of an emergency.

Bear in mind that while emergencies are rare, when they do occur, they are serious and the crew have safety jobs to perform. Your father, in consultation with you, will need to make a determination as to his capabilities and whether the stress of the trip alone is a net negative. There are 89 YOA's who travel worldwide on business all the time. There are 30 YOA who can't walk to the corner alone.

If your father is OK with a wheelchair to the gate and someone helping him to stow his bag, that is one thing. If he needs attention in turbulence when the crew is seated, he needs to travel with a companion.