It sounds like your son's school has handled the transition very badly. Frankly, this year's 3rd graders should be some of the best supported for the transition because K-2nd grade aren't testing years under NCLB. The standards were released at the beginning of your son's Kindergarten year. Schools, school districts, and states that have handled the transition well made changes for this age group starting in first, so that they were prepared. They had the ability to do so because under NCLB and RTTT schools aren't held so tightly to standards for K-2. If it feels like an abrupt transition for a 3rd grader, then that's really problematic.
The other thing I wonder, is whether some of what you're seeing with your son has to do with the ways that specific characteristics of your child, mesh with the demands of 3rd grade.
I work with parents with challenging kids, and I often remind parents that every kid has weaknesses, and when your child who struggles harder than most with something, gets to an age when all kids struggle with something, it's likely going to be a rough phase. If you have a girl who tends to be moody and defiant, 3 is going to be rough, middle school is going to be hell, and you should be prepared for the stress of their senior year in high school. If you have a boy who struggles with impulse control, and sitting still, his twos might be terrible, you should expect some negative feedback from his K and 1st grade teachers, and prepare to need to hold him close and give him a lot of support when his body is flooded with testosterone from his teenage growth spurt.
You mentioned upthread that your child seems to struggle with fluency. You said something along the lines of he can do it, but it takes a little more time. If that's a stable characterstic of his 3rd grade is likely to be a tough year, because the 3rd grade curriculum, before CC, and with CC, places a lot of emphasis on fluency. Kids are expected to learn all their multiplication and division facts (in contrast the addition and subtraction facts are spread out over 3 years in K-2). They're expected to do a lot of mental computation. They're also expected to work with tasks that require them to use the four basic operations in the context of a multistep task (e.g. finding elapsed time, or the area of an irregular shape, or comparing frations). Kids who seemed to be keeping up, but took a little more time, can suddenly feel swamped, because while they can do each step, the steps aren't efficient enough for them to be able to hold on to them while also looking ahead to the next step and remembering the number from the step before.
The good news is that he's growing and developing, and if you and the school support him well he'll catch up. The demands of 4th grade math are very different from the demands of 3rd grade math. The pace slows down again to allow kids to delve deep into new concepts, like decimals and angles, and for your kid this might well be easier. Plus he'll be growing and maturing and when demands are placed on his fluency, he'll be more ready to manage them.
Good luck.
Personally, I don't think "please excuse my dumb algebra students" is mean. I think it's funny. I once was and still am a dumb algebra student and can laugh at the fact that knowing I am helps me remember the order of operations. My kids thought it was funny too. Both a great in math.
