Again, it really depends on what the necessary features are. The Yamaha should work well for him, however if he really wants to start getting into creating music via sequencing, then he will "grow" out of this keyboard.
The Yamaha does have a headphone connection, and it also has many more tones than the Casio. In regard to the tones, the Yamaha is much better. Has many more to chose from and they sound realistic.
Within the price range, the Yamaha is a great keyboard. In regard to your question about what it doesn'
Sorry...what it "doesn't have..."
That depends what you are comparing it to. It has many more features than the Casio - actually you aren't really comparing apples to apples with the 2 keyboards. The Yamaha really offers much more. But back to what the Yamaha doesn't have. It doesn't have very good sequencing capabilities, such as a 16 or 24 track sequencer. It is not really a "synthesizer" that can be programmed. These would be more like the Yamaha Motif's or the Tyros. These are professional keyboards and are very expensive. I think for your price range, you are getting many features verses other keyboards in that range.