Well I would just learn to play using lead sheets, such as EZ play today books. First find what type of songs he would love like to play, start off with slow songs, hymns are mostly in the range. If he does not mind playing children songs such My Grandfarhers clock which sounds slow or fast.
Ez play today do some good children ones.
The keyboard has what is called one finger chords, which is a good place start, unless he whants to play all sorts of music in depth which can take years. Having played some songs, he could learn full chords starting off with C, G, F. Lead sheets are just Melody Right Hand with Chords in the Left. So learn the Right Hand then learn to play left hand, single finger or full chords.
Also use the styles etc.
Lots could be said but play slowly at first and learn the names of the keys.
All the best
Now for the following I am in the main talking to older adult, every journey starts with one step, in the case of learning the piano starts with one key. So in the very beginning just understand where middle C is. Then where every C is on the keyboard.
So you have C (middle) D, E, F, G A, B, C This pattern is the repeated cross the Keyboard. This is the most simple way for a beginner. Working with the Right Hand from Middle C, the pattern is of course repeated on the left side of middle C. For the beginner just be concerned about the Right Hand at the start. Any piano book should show you the fingering staring with the Thumb as 1, next finger 2 and so on to the little finger as 5.
C. D. E. F. G
1. 2. 3. 4. 5
That does not mean that every D is played with finger 2 it just means every finger is numbered starting with the Thumb as 1 …… 5 as the little finger. Every piano book shows this relationship.
Ok now place fingers of the right hand over the keys and play from C to G one finger at a time, and try each key played the same time interval apart. Let's say at between 1 and 2 seconds so C is played 1 second later D is played, next second E is played etc. Play slowly at first the try to increase the tempo. You may well find when trying to play fast you make errors, that play at different times or miss a key. That is good we learn by our mistakes. Just find a speed you are happy with, and only increase your tempo in baby steps. At this time when you can do this with out errors you are ready for the next step no matter what your tempo is. Fast Tempo will come in time best to play at your own speed.
One way is to say one, and two, and three, and four, and five
Then 1 2 3 4 5
I could go on but what I have stated maybe you can already do so if you can that's great. I am not a piano teacher but am self taught and of course a teacher is best, but they cost money and books for me do not always show what I need and when you a teaching your self you need to know what you are looking for. The most simple books are best, not necessarily the most expensive.
And as I have already stated EZ play today are for me the best for Adults and those that play from a lead sheet.
A book which I can recommend is Teach Yourself to play Electronic Keyboard ISBN-10 is
0-88284-680-9 by
alfred.com get it on Amazon, EBay etc.
The next step is to learn the scale of C Right Hand only. You need to understand thumb under/over etc.