I reckon that the answer for your Dad is for you to get him a Chromatic harmonica. The scale is identical in each octave. That means that the blow and draw takes place in exactly the same place no matter where you are playing, high or low octaves. No playing in a different key to that of the harmonica,which is what is required on a diatonic tuned harp. There are many makes and different keys available, not all are horrifically expensive. The only thing required to play them well, is to get used to playing a single note at a time, and not a mouthful of them! This comes with practice, and you can get a full selection of nuances , from sweet to harsh, if you persevere. The ability to play semitones (sharps or flats) is permitted by the use of the button on the harmonica, and all of the notes you can find on a full piano keyboard are available to you when required. Great if you want to play some jazzy runs and slides!
I hope this helps, Good luck