Sorry for the delay in replying.
I can't say if he's learned fast or slow, I can say however...he loves it!
Whatever he is being taught has given him the confidence to start finding how to play songs he enjoys on the piano himself.
His most recent accomplishment is a one hand version of the theme from Jaws, leaving the other hand free to feign the fin of the attaching shark.
I know that this is most likely not the way Horowitz was taught, but my goal is to help him find joy in music and with that in mind, I'd say it is a great success!