Welcome, Bea!
I will chime in with another "it's not your son, it's the environment" post.
I can understand the "too immature to handle the workload" argument, although I think it is somewhat bogus. There is a certain level of executive function skills necessary to do some higher grade work, that younger kids may not have. But if I had listened to the people who advised me to have DS9 repeat kindergarten because he wasn't mature enough for first grade, he'd still be in kindergarten.
We wound up taking both our kids out of school. For DS9 (also PG), he became a behavior problem (defiant, disruptive, meltdowns, etc) in 2nd grade that the school was unable to address. DS has still not recovered fully, and I regret keeping him in school for as long as we did.
DD7 kept her excellent behavior in 1st grade, but her schoolwork started disappearing (she later told us she was dumping it in the trash). At home she would have meltdowns and mostly refused to do homework. Not wanting to make the same mistake twice, we pulled her out soon after starting 2nd grade.
Your son is doing the best he can given a lousy situation. Imagine having a job that required you to go to all-day meetings at work, with all the powerpoint slides covering topics you already understand, aimed at people less intelligent than you. You have to fill out worksheets and answer questions based on the presentation. You can't check your email or play Angry Birds on your phone. You have to appear to be paying attention, all the time. No matter how good your work is, you have to go back the next day and do slightly re-worded worksheets on the same topics (and do the same at home when the workday is over). Most of your co-workers appear to enjoy, or at least tolerate, these meetings; some are struggling to understand the information presented. And you can't quit, because, well, you're not allowed to.
Why the heck would you bother expending effort and trying to do a good job, since doing a good job doesn't result in any meaningful change in the situation?
The previous posters have offered some excellent advice. Your son needs to be in a different environment.
