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Is there any advantage to him learning two instruments, or should he just stick to the saxophone?
Chris, if he's enthusiastic, then learning a few instruments is fine. Most people seem to play at least two, often more, but generally have their favourite. My favourite is definitely clarinet, but I've played lots of others!
Apart from the cost, the other consideration has to be time. Playing musical instruments is like playing computer games. The time goes by so fast!
Going from the clarinet to the sax is dead easy. The sax embouchure is a more relaxed version of what's used for clarinet, and the basic sax fingering is the same as the clarinet clarion (2nd) register.
Going from sax to clarinet would be harder, but at least a lot easier than for a beginner.
Just a word about buying an instrument. There is glut of second hand clarinets on the market - and saxes as well. With the increasing number of chinese imports, there's no shortage of cheap instruments.
I would suggest buying a well know make. The resale value will be much higher. Yamaha saxes have a great reputation and will always sell easily and for a good price.
Good luck!
Steve