I've been playing the piano / keyboard since I was 6, I'm now 34. If it was my son I'd be looking at picking up a relatively cheap digital piano.
Here's a few things to look for:
- 88 keys that are at least semi-weighted, if not fully weighted.
- A nice range of voices, most cheap digi pianos will come with 6 or more.
- A set of MIDI connections, or a USB connector for hooking up to a computer is handy.
- Decent polyphony (this is the number of notes that can be played at once, I'd say 32 as a bare minimum).
I'd avoid an acoustic piano simply because cheap pianos will always sound cheap, also if you have central heating it will never be in tune for more than a few days. A full refurb on a piano is an expensive job and usually not worth it., however if you do find Bechstein, Steinway or Bossendorffer going cheap, grab it.
As with most things how much you're willing to spend will determine the quality of the board. Some manufacturers to look out for:
- Yamaha (Clavinova)
- Roland
- Technics
- Kurzweil
Be careful when talking to a salesman and make sure you specify a "digital piano". An Electric piano is a different kettle of fish all together as is a stage piano. To be fair I didn't use a fully weight 88 keybed until I'd been playing for a while, I started with a cheap 61 note keyboard, then a 76, then bought a decent acoustic with 88. (All acoustic pianos will have 88 btw).
61 notes will be enough to learn basic chords, melodies and the fundamentals of pitch etc but will be no good when learning to play "actual" piano compositions, for that you'll need the full 88. Rachmaninoff used 88 notes and he was the best, so y'know... ;-)
*EDIT* just realised I've basically repeated what
@JtB said.