"Classic" autism really only signals the early onset and enough symptoms from all 3 categories. If you eventually suspect he's not on the spectrum at all, then it's worth re-evaluating at a later age.
For me, the diagnosis placed my son correctly on the autism spectrum. He may move up or down on that spectrum, but I don't think he'll ever get off it. Even though he might learn to "pass" he'll always have the disability. If the official label eventually causes problems with school or public services, I will try to get it changed. In other contexts, I feel it's up to me to convey accurately where he is on the spectrum, so I'm free to ignore the official label.