Looking to buy my brother a piano and he's pretty much a beginner but learns fast. What kind of piano can he learn on and continue using even after he's learn a lot?
You have three broad choices - a real piano, a self contained electronic piano, or a MIDI keyboard that can control a computer.
The most desirable pianos are grand pianos, but of course they take a lot of space. Upright pianos are a very good compromise, and take somewhat less space, although it's still quite a chunk out of a small apartment.
You're unlikely to find a real piano that's not suitable for learning on. They only issue would be a second hand piano that's been very badly looked after -- dried out or rotting to the extent that it can't stay in tune or the tone is awful.
Different pianos have different keyboard feel, but as a beginner your brother won't have developed any particular preference.
Limiting ourselves to instruments that at least aspire to sounding like a real piano, there are a range of instruments, ranging from free standing ones with a similar size to an upright piano, to smaller instruments designed to be placed on a keyboard stand or a table. Choose according to space and budget.
As Matthew Read says, there are unlikely to be any serious instruments on the market today that have a horrible sound. Broadly speaking, the more you spend, the more sophisticated an imitation of a piano it will become, but that kind of detail is probably wasted on a beginner.
Electronic pianos have the advantage that you can play through headphones, if you don't want to disturb others.
They also usually have a variety of piano sounds, as well as many other sounds. Also they tend to have features such as recording sequencers, drum patterns and auto-chord accompaniment. This can be seen as a disadvantage, as you can get distracted by playing with the features, rather than learning to play piano.
Most electronic keyboards can also be used as controllers for a separate sound source, should you wish to use them as an expensive MIDI controller.
Here I'm referring to keyboards that can't produce their own sounds. Rather they send signals over MIDI or USB to a computer (or a standalone sound module).
The advantage of these is that (if you already have a suitable computer) you can spend your money on a nice physical keyboard, rather than paying again for a sound source. If you become more serious, and want a better piano sound, you can later buy a more sophisticated piano sound for the computer.
By virtue of doing one thing well, these are generally not much larger than the actual keyboard, and hence are easy to carry and store. A disadvantage is that you have all the overheads of setting up the computer/module before you can play.
Whatever kind of electronic keyboard you get, here are some considerations:
Most any standard piano will do, so I'm assuming you're talking about an electronic one.
I think the main thing is having regular size keys and a full 88-key keyboard. He won't use all of it at the beginning but if he does progress fast then he will soon need it.
Another thing to look at is the touch. You can't learn to play dynamics properly unless the keyboard's volume is proportional to how hard the keys are played. A related problem that a lot of keyboards suffer from is that they're too easy to play. This makes it hard to play quietly and impossible to build up the strength needed to play a "real" piano. They also have an unpleasant thwacking noise that occurs when the keys smack down.
A pedal is useful.
I don't think there's much else. The sound isn't that important once you get past the ones that sound really electronicky, which are rarer nowadays.