Your son is in 1st grade -- full day. A whole day where he gets feed back from a teacher who doesn't understand or know what a gifted kid is like.
Gifted children are asynchronous. Not everything develops at the same speed.
I've heard many parents on this board and other places, Jan-Feb is when things start to fall apart for a lot of kids. They may have been excited to go to school and learn in Sept, Oct, but then they get negative feed back about their work and they aren't really learning much. It is really tough on the kids at school who are advance.
Just a thought for you. My kid scored better with the Gr. 3 reading material and answered comprehension questions better because there was more meat to it. The Gr. 2 material, my kid only did so-so. Good thing our psyc-ed tester knew to keep going and see what would happen.
Try it. Give your son at home something more challenging and see what he thinks.
Do also read up on perfectionism as that is often an issue with gifted kids and it starts early. Gifted kids often judge themselves harshly expecting themselves to perform at a higher level.
Best wishes! Let us know how it goes...