My son has high functioning autism as well he is 8, and it sounds like him and your your young Zach are from the same groove.
You descibe him so well, and I see alot of my son there.
My boy is also incrediby sweet, and comes out with the funniest of things sometimes. He is a real cuddlebum with me, for some some reason cant stand it when anyone else tries to kiss him or cuddle him.
My son is a whizz on the computer, teaches me how to do alot of things on it, and then is sooo proud of himself becasue he can teach something, he goes around telling anyone who will listen what he did. He does have the occasional meltdown, but we always use distraction to try and head it off before it gets that bad. He is in a mainstream school but also has an aide to help him to understand. His writting and spelling is not that good, but he loves to read, and is already reading novels on his own. Typing on the computer he is amazing as well.
I love my boy to bits, and I wouldnt have him any other way. He is very special to me in so many ways, with his quirky behavior, which he amplifies when it makes me laugh. All he seems to want to do when I am around is please me and make me laugh which he strives to do. The best gift I can give him is to laugh at his jokes, or pranks (which he plays on my DH all the time)because it makes him soooo visibly happy. Material things mean nothing to him, but give him a smile and you have just given him the world.