I am not so sure there is an easy answer to this one Oakleaf. I am assuming (perhaps wrongly) that an Arts College has a slightly different focus than an normal college or school. I would look for the positive in this, it may help with his confidence and expression, give him a more diversified outlook and approach, these are things that would definitely benefit him in the eyes of an employer and could be considered 'another string to his bow', sorry very bad pun I know.
How does your son feel about learning this instrument, does he feel the same as you? If so then perhaps a change of school/college should be considered.
I do understand your concerns but as long as he is getting tuition in other more 'academic' areas I personally don't see the problem.
As an aside, the school I attended taught me Latin and Greek for 2 years, and to this day I have never used them to an 'employment' benefit, comes in handy in pub quizzes though. I, like many of my peers, thought it a waste of time and very unenjoyable and would rather have had african drum lessons in their place.