I'd ask him why he feels that way - that's my default answer whenever I'm thrown for a loop when my kids ask a tough question! Sometimes the reason they are asking comes as a complete surprise, or is much simpler than what I would have thought it would be based on the question.
Then if he's asking because he's bored etc and wants more challenging work, I'd be honest with him about things - if he doesn't have an option of moving up and yet he really wants to. Explain that the school won't let him change grade levels, but offer him other ways to put some interest back into his classwork or offer him the opportunity to do some extra studying on something he loves outside of school. It's far from ideal, but it's something. Another tactic I've used with my kids is to take them deeper at home in the subjects they are studying at school. There's a potential to "going deeper" and after-schooling ahead in early elementary if you keep a record of it because you can use it to advocate for acceleration and higher level of challenge as your ds moves up in school. Early elementary was tough for our ds, but as he moved into middle school there were more options for ds, and having the advanced work he'd done after schooling in elementary school helped move him subject-accelerate (at last lol).
The other thing I always do for my kids when they tell me they have something about school that is bothering them is to let them know I'll talk to the teacher about it if they want me to. If they do want me to, I also ask if they'd like to be there when I talk to him/her, and I honor whatever there wishes are re that. Then I follow-up and tell the teacher about how my child is feeling - even if I know it's not going to accomplish anything or fall on deaf ears - the important thing is that I'm showing my child that I tried, and in the process he'll begin (over time) to learn to advocate for himself.
Last thought - have you talked to your ds yet about his ability level? I found it really helpful to have that conversation in early elementary when our ds started getting noticeably bored and frustrated with the subjects he excels in and wanting more challenge. It helped for me to talk about how different people have different talents (sports, music, etc) and how different people have different levels of abilities in sports, music etc; then we talked about how one of his "talents" is being able to understand concepts quickly and I introduced the concept of a bell curve to him and explained where his intellectual ability was on the curve - and that was like a lightbulb going off for him (at 6-ish) because I think he, at that point, was also getting frustrated about friends who didn't understand a lot of the things he was interested in.
I think I'm rambling now - hope some of that helped!
polarbear
