Ok, a few things to take into consideration.
Weighted (think piano) or unweighted (think organ) action. You have to be very clear on what he needs. If he has a piano background, he probably is used to a weighted action, but just be super clear on what the need (desire) is. It is not unheard of to have one of each. A nice weighted unit for playing piano and Rhodes parts, and a light-weight unit for organ and synth parts.
The feel can also be very personal. What feels good to one player may be almost unplayable to others. Unless this is to be a surprise gift, you should get him to play a bunch of different models to get an idea of the range so he can get what is good for him.
As a main controller, you need to consider the features other than keyboard as well. Having aux controllers available that are easy to route can be a very nice feature to have.
Don't overlook the need for a good stand to put it on, unless he will always be playing at home. a good quality stand can be one the the most important things a keyboard player can have.
Will this mainly be for home/studio use, live playing or a mix of both? For purely home/studio use, a controller can be very nice, but once you start to play live, it adds a level of complexity that can call into question whether it might be better to get a good quality instrument that makes it's own sounds and can also function as controller for other sounds modules.