If you want to play piano then an electric keyboard is a poor substitute, but equally, if you want to play electric keyboard, then I think a piano is a poor substitute! They are different instruments with different capabilities. If he's drawn to the keyboard (eg seen it being played in a band?) then that seems a good choice.
It sounds from your post like he wants to explore this to get a better understanding of music in general / enhance his singing and musicianship, rather than a specific desire to learn piano. In this case, I'd venture the opinion that keyboard is a good option. On a keyboard you get a whole bank of beats and sounds to explore to incorporate into creative music-making.
The other thing to bear in mind is that I believe classical piano and electric keyboard tend to be taught in different ways by teachers and tutor books (ie it's not just that the music is of a different style, the approach is actually different). Classical piano tutor books focus on note reading in both the treble and bass staves. Keyboard tends to take a chordal approach (more like jazz piano) - you learn to read notes from the treble staff and chord notation so that you can learn how to play around with the chords underpinning the music. Again, if his objective is to support his singing and learn how music is constructed, I wonder if the keyboard / chordal approach might be most useful. As well as being able to read music, another important musicianship aim is to get a good understanding of harmony. Eventually with classical piano you do this too, but it seems to me like the keyboard approach puts this to the fore because of its chordal approach. (Speaking personally, despite years of classical instrumental lessons as a child, my understanding of harmony and how music is constructed was poor because all I did was read the notes in front of me.)
But I've not answered your question yet. We have three keyboard instruments: A great Yamaha PSR-E363 keyboard which has so many functions and would fulfil any beginner keyboarder needs I think (think that was about £150). We also have a Yamaha Arius YDP-164 digital piano (I only play that as I much prefer the sound of a piano, even though I tend to play 'keyboard / chordal' style - that was £900). And we have a Yamaha PSR7 420 which came from a charity shop for £20. My 7-year old will only play the cheap one! I'd say that you don't really need to spend much to get an instrument on which you can learn to read music and understand keyboard harmony. My DS will be able to achieve this on the £20 instrument (if he actually plays it of course!)
For keyboard tutor books DS has the Progressive Keyboard books which seem good and I think could definitely be used to teach yourself. To be honest we don't follow the books much - more exploring the chords and improvising.