First, allow me to express my envy that you even have full-grade acceleration on the table as an option. Then, let me express a cautionary tale, because it's very possible that full-grade acceleration may not be on the table as an option next year.

Ultimately, you have to figure out who your son is, and how he's best served, but as a 6yo who has already demonstrated he doesn't belong in K, I'd say a grade skip is a great option.