Ike - unless your dad is way younger than I am (70), then I suspect he either wants to shoot Cowboy Action or shoot as if he did. You need to give old men slack on that.
There are two reasons why Trail Boss is, IMO, the premier powder for Single Action Army replicas, particularly guns with fixed sights because they are far less likely to be used for serious hunting:
1. It is an intentionally fluffy powder that fills the case not far below the level of the bullet seat. It appears to be impossible to double-charge a load without spilling powder all over the place. This is important because older men, in the company of buddies, tend to BS a lot while loading. Even alone, it is important, since a double-charge is so readily visible. Regrettably for the entrepreneurs, it torpedoed the dreams of the forward-thinking folks who invented the shorter .45 Cowboy cartridge, which would use ball powder and similarly prevent double-charges, while dealing with #2 below.
2. Shooting holstered revolvers is very different from shooting lever rifles, at least in CAS, because the revolvers have to be presented from a barrel-down position and the rifles are presented from a barrel-up-rotated-to-table-level position. Same load, in most cases, but for in revolvers with tight low-grain loads there can be an inconsistent powder distribution in the cartridge and relative distance to, and spread from, the primer. For example, Titegroup and Clays use approximately 2/3 the amount of powder by weight, and probably half by volume, as Trail Boss. Not saying don't use them - they are popular with many top dog CAS shooters. Just sayin'.
For old guys like me, who shoot just for fun and for great memories of glories past, without reservation I recommend Trail Boss for your Dad.