Excellent news that there has been so much progress!
One serious word of caution - some elementary schools are notorious for transitioning high functioning special Ed kids out of special Ed before midschool. They tried to do this with my son as well. Then once they hit midschool with all of the new transitions, larger classes, multiple teachers ... these kids can founder without the supports of the IEP.
Is there a way to not transition him off the IEP but have maybe only one intervention of having a support person check on him once a week or a month - just so the legal document is still in play in midschool?
I've found the teachers in midschool to be less tolerant of the quirks associated with Aspergers - the literal interpretation of rules, etc.,
This has been my general experience as well. Middle school can be like a dress rehearsal for The Lord of the Flies. Rather than cancelling an IEP, perhaps it would be a better idea to put all his therapies and ESE input on "consult" until the first semester of sixth grade is over. We generally call it "transition planning", especially necessary given the average Asperger's kid's success with transitions.
