BTDT!.... You sound just like me, oh about a year ago. My DS made a radical jump from K to 4th, crashed and burned. We homeschooled for a year, then he was accepted into a small charter school that focuses on math and science.
He is now enrolled as a 7th grader at the tender age of 7 and it couldn't be going any better. I think a lot of the success of these kinds of jumps is determined by the child himself and what he wants. My DS could care less about the social aspects of age mates so for him to be surrounded by older kids is fine. The first jump was done without his input. The second he was all for and it made a huge difference.
As far as the IAS goes, it does not recommend multi grade skips at all. I called them and asked that specific question. They think that in those situations a child should skip one year every year ie: K, 2nd, 4th, 6th, etc...
