How do I report a distribution I received from my son's 529 account for eligible college expenses? He attends a military academy. How is this taxed?

If the funds were used for qualified education expenses you don’t have to report it. 

If he attended a U.S. Military Academy (USMA), he doesn't have many, if any, eligible expenses.  Most of the earnings portion of the distribution (box 2 of the 1099-Q) will be taxable. However, attending a USMA is an exception to the 10% non qualified distribution penalty.

You may need to use a workaround to enter this in TurboTax (TT), if your son is still your dependent.*

In the 1099-Q section of TT,  when asked who is the student, check "someone not listed here" (Lying to TurboTax to get it to do what you want does not constitute lying to the IRS). On the next screen, enter the real student's name.  This will eventually give you one simple screen to enter any expenses. Press Done at the 1099-Q summary screen, to get there.  Be sure to also check the "attends a military academy" box, so that TT prepares form 5329 to claim the penalty exception. 

*Military academy cadets do not usually qualify as dependents, except possibly the first year.