That doesn't really sound healthy... why don't you tell him to sit down and play in a relaxed position. You should not slouch, but still keep very relaxed.
The clarinet is a bit harder to play than saxophones etc because you have to put more pressure upon the reed and blow more to make a sound. He could be having trouble because he's using a to hard reed, its good to be starting with a 1 1/2 or so and as you play more you're embouchure develops more and you use harder reeds.
Does he take music lessons? It's probably best to have some when starting so as not to develop bad habits and a teacher might be able to help the 'straining' problem.
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