If your dad doesn't want to hear about water conditions and health (many old-schoolers think a fish should be able to live in a shoebox-sized tank full of turds and be fine), try explaining to him that any one of these fish normally claim a territory the size of a typical coat closet in the wild. So while a 50 gallon tank is marginally enough space for one of these individuals, it's not going to cut it in the long run for more than that.
Bluntly speaking, Oscars should be kept in at least a 75g to make nitrAtes easy to maintain below 20ppm. So try to get him to like the jack dempsey as it's the most suitable inhabitant of the stock he has now, and to return the others.