Christine, my answer falls somewhere in the middle. ;)
I'd offer a graduation date of next December/January BUT only if he had made the necessary plans, taken the tests, completed the work and arranged transportation. If he isn't open to a long discussion, simply let him know what you expect and need to plan yourself. And do give him a deadline. This option would allow some time for maturity.
fwiw, Looking back on my graduation at 17, I'm not a fan of early graduation.