Your DS is going to be skipping one year no matter when the skip takes place. He is likely to miss less substantive curriculum by doing a skip mid-year. I really would not worry about some end-of-year test, especially if it is meant to be at level. Realistically for most high ability students, they should not have trouble with curriculum a year or even two above. The only thing that I would do differently is to let your DS in on what is happening sooner rather than later. I don't think he should be the last to know as I would consider his opinion as important as yours or the school's.