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I guess I just would like to know what to expect? Is he going to be expected to learn the ABC again?
While teachers, programs, and schools may differ... in general children are most often expected to reinforce skills learned/mastered long ago and wait for same-age peers to catch up... rather than being provided appropriate instruction, curriculum, and pacing, matched to their ability and readiness. This is where advocacy comes in. You may wish to read all you can about advocacy, and keep lists of your child's accomplishments including a list of books read. Collecting selected works in a portfolio can also be helpful over time.

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I havent told the school he is gifted beause I dont want to place expectations for him to live up to, but they are going to know as they have requested the WPPSI in order to work out what special education requirements he needs.
Is this a public school? In order to close the achievement gap or excellence gap, some schools may define "free and appropriate public education" (FAPE) as having all children working at the norm for their chronological age. These schools may not endorse a goal of having each child working at his/her ability/readiness, if the child's ability/readiness exceeds the norm for his/her chronological age. These schools may tend to conclude that if a child is working on par with same-age peers, or is ahead of same-age peers, than no accommodations or supports are needed because in their view the child's ASD (or other LD which causes the child to be 2e) is not "impairing" their access to what they deem appropriate education. For information on advocating under these circumstances you may wish to see the wrightslaw website and read the book From Emotions To Advocacy.