No, there is no tax deduction or credit for that.
At most, a musical instrument required for a course of study might qualify for the American Opportunity Credit, but that credit is usually maxed out by baseline tuition, and is not available for Masters' students in any case. Even if you could still claim your child as a dependent (which is unclear, but probably not), a musical instrument would not qualify for the Lifetime Learning Credit.
If your son becomes a professional musician and reports self-employment income, he can list the instrument as a business asset and claim depreciation, using your cost as his adjusted cost basis. (Technically, whenever he starts earning money as a professional musician, he places the asset in service and depreciates it using whichever is lower -- the cost you paid, or the fair market value at the time he places it in service.). (Note that depreciation can only be claimed if he is self-employed, not if he is a W-2 employee of an orchestra. If he has both wage income and W-2 income, he can allocate a portion of the depreciation to his self-employment.
