I wouldn't hesitate to do that. Far better to have a few hours over the summer than a year of stuff he already mostly knows, and then he'll have a good class in the fall.

DS11 in 7th grade this year had 8th math for the first half of the year and then we moved him into e-school Algebra I for the second semester, and he didn't even blink at starting in the middle of the year for that class.