Ok, I have a few thoughts on this one. Don't just treat the problem, treat the cause. In addition to not being challenging/or too challenging, how is he being motivated? Grades are kind of an abstract thing at his age, so I would recommend a more immediate, tangible motivator. It can be something small like at the end of the week you and him going to get an ice cream cone to talk about school, or something bigger, like a vacation somewhere if he has a great year. A lot of people frown on motivation, and I don't really understand that. The way that I see it is that it takes motivation to do everything in life, whether it be internal or external. Think about it, would your husband go to work every day if he didn't get a paycheck at the end of the week? No. Would you clean the house if it didn't give you some satisfaction or peace of mind? No. Try making the trip to school more fun, play I Spy, the ABC game, plan something fun to do after school. Just do something to make the way to school as fun as possible, so he will associate going to school with enjoyment.
If you talk to the teacher and he is in fact not being challenged by the curriculum you probably don't need to do anything as durastic as changing him schools right away, but start small. While his classmates are working on grade appropriate materials have the school provide him with the next grade level up materials. If that isn't an option look into a combo class (1st/2nd), that way he still gets the socialization of kids his own age, but is able to get the instruction he needs. If he is struggling look at some of the possible reasons why. A lot of people associate ADD with ADHD, but in fact they are very different. I am not saying or suggesting that your son has either, just giving an example here. I worked in a classroom with a little boy with ADD struggled a lot, and he was very slow with his work, and had a hard time focusing, but was not hyper at all. Also, the curriculum itself may just not offer any emotion. This is a little bit in depth, but certain curriculums out there are designed on a script, and its up to the teacher to take these materials and enhance them to make them exciting for her students. If your son goes to a school with these curriculums its no wonder he's bored, they are boring to teach.
I personally would not recommend going into abstract explanations about why he should be happy to go to school, you would probably be wasting your breath. If this is happening frequently, where some weeks he loves school and some weeks he hates it there is something going on there. You get to play private detective to figure out what the difference is (lucky duck!) Don't give up hope, school may never be your sons favorite thing, but it doesn't mean he can't still grow up to be successful. Good luck and just keep trying :)