I third that. I think this isn't really a multi-grade skip. Not really. He's already done the heavy lifting of said skip this year. All you're doing is making it official. BIIIIIIIG difference!
I say stop thinking of it as a multi-age skip and just do it! I really think it's okay!
And you know, if it turns out to not work for some reason that I can't fathom right now, you could deccelerate him, right? I mean, the school seems sooooooo supportive, I think you have alternatives if they turn out to be necessary. (Though I REALLY don't think they will be, given what you've told us.)
