DD couldn't read when she started school, but she could write all letters, and had a goog grasp of numbers. She is now starting to read, and is writing some things independently.
I think the main thing she has learnt is a little independence at school, how to settle, the school routines, and how to be really happy at school.
I go in to help once a week, and have observed that a lot of the learning is oral, there is a lot of class/teacher discussion, and small group work, which may well not have a recorded outcome.
My DS is now at the end of year 2, and he sounds a bit like your DS, he could read fluently etc., before he went to school. Tthe work changes in Y1, to more obviously structured and formalised, with more work written down, and lots of topics which DS has inhaled, and explained to us in much enthusiastic detail.
Hope that helps a bit