I have for years used 529 funds for the education of both my son and my grandson. Why is TurboTax telling me my (nondependent) grandson is not eligible?

There is no place where TurboTax (TT) tells you that you are not eligible to use a 529 plan for a non-dependent beneficiary. It may not let you enter educational expenses in the "regular" place [educational expenses and scholarships (1098-T)]. For a non dependent student, the expenses are entered at the 1099-Q section. Press Done at the 1099-Q summary screen, to get to the expenses entry screen.

Better yet, 

You can just not report the 1099-Q, at all, if your student-beneficiary has sufficient educational expenses, including room & board (even if he lives at home) to cover the distribution. When the box 1 amount on form 1099-Q is fully covered by expenses, TurboTax will enter nothing about the 1099-Q on the actual tax forms. But, it will prepare a 1099-Q worksheet for your records. You would still have to do the math to see if there were enough expenses left over for the parent to claim the tuition credit. You also cannot count expenses that were paid by tax free scholarships. You cannot double dip! 

References:

1. On form 1099-Q, instructions to the recipient reads: "Nontaxable distributions from CESAs and QTPs are not required to be reported on your income tax return. You must determine the taxability of any distribution." 
2. IRS Pub 970 states: “Generally, distributions are tax free if they aren't more than the beneficiary's AQEE for the year. Don't report tax-free distributions (including qualifying rollovers) on your tax return”.
3. ("IRS Publication 970, Tax Benefits for Education states: If the entire 1099-Q went to qualified expenses, room and board, tuition, etc then you do not need to enter the form."