Yamaha P140.. able to connect to Garageband?

Would he be able to use this keyboard as a midi controller through GB without a USB out?

he needs a MIDI interface:

http://www.bulletsandbones.com/GB/InterfacesMIDI.html

How do you disable the sounds from the keyboard

there's likely a "local" switch (or something labeled similarly) to toggle between Local and MIDI controler

You need a MIDI interface - you can buy a simple thing for not too much money that will give you MIDI in and out (although the out isn't usable with Garageband) and will plug into a USB socket.

If you might want to record with a microphone or instrument in the future, then it might be as well to think about it now and buy an audio interface (most of them include MIDI in/out as well).

There should be a setting on your keyboard called 'Local off' which disengages the direct input from the keyboard but sends the MIDI out for just such a situation.

Oh yes. The MidiSport from M-Audio are great. If GB doesn't see or here it, you may want to get the 1x1 version, because I KNOW those are compatible with GB.