Trina, how awesome that the school is so co-operative, acceleration plus part time! Hoping, based on how co-operative they seem to be, that the school will carefully support his unusual start date, start point and age, and given your son's scores and achievement data I would cautiously give it a go (not speaking from personal experience here!).
Personally I would not underestimate the amount of learning that needs to happen that is NOT academic. He may take longer than you think to settle into school life, or he may power through. You may find that there are some unexpected road blocks, such as handwriting, ability to focus on boring stuff, ability to keep himself organised enough to function in higher grades. You may find that what you need is this jump start plus some subject acceleration whenever he shows that he is ready.
Have you read any articles about Terrance Tao's schooling in Australia? If not you may find them interesting and helpful both he and his parents have talked very openly how he progressed, how they made decisions, what worked and what didn't.
