ETA:
After reading your SWH, all I can say is, UG! How unbelievably frustrating! I am so sorry.
You do have some academic concerns, but it is almost impossible to tell whether or not any of these could be easily addressed. Please keep in mind that many of them actually could be easily addressed if you were able to communicate with the teacher. Sometimes all kids need is a little bit of extra help to get over a hump and they are off to the races with learning. This really could be the case with your son, but if he is frustrated and school and not getting any help with it, he is definitely not going to want to do homework. He may or may not have ADHD or any other concerns.
Right now I think you have to call the principal. I hate saying that. I really do. Because I am a firm believer that you should always try to address things with the teacher and not go over the teacher's head. But in this case, you've already done that. I think you need to go to the principal and say, "Help me! Help my son!"
Please don't worry about him "failing." I don't even know why some schools still use grades at this level. I am very thankful that my 4th grader still does not have actual grades. Our district doesn't use letter grades until Middle School. Letter grades don't quite reflect what a child is learning or what he/she is capable of. They reflect whether or not that child completes assignments and turns in homework. That's very important in middle school and beyond, but not so much in second grade.
What you what to focus on is whether or not he can read at grade level and do math at grade level. He should be receiving speech services through the school (which means an IEP). His progress there should be a part of his report card.
I could go on and on, and right now I don't even know that you'll see my edits :-)
I encourage you to speak with the principal and set up a meeting with you, the principal and the teacher. These concerns need to be addressed right now! I'm not too worried about your son. He has lots of time to catch up on any areas that he is struggling with. But the longer he struggles, the harder it will be for him to have fun at school and like learning again and the easier it will be for him to be discouraged.
Call the principal now and start there.
Original Answer:
My son was almost 6 years old and in kindergarten. But he doesn't just have ADHD. He is also on the Autism spectrum.
If you suspect something is off, you need to consider a few things: does he need more sleep, is he eating well, does he get enough exercise, does he get enough down time and play time after school or is it just homework, is he expected to do too much homework.
Have you discussed your concerns with your son's teacher or with your pediatrician? If you truly believe it's more than just diet/exercise/sleep, etc, I would suggest asking the school for an evaluation and talking to your pediatrician. We did both.
We had our son evaluated by the school. He did qualify for resource time and more speech time.
We also talked to our pediatrician who referred us to a psychologist. Our son first saw a psychologist when he was 4 years old. He wast diagnosed with ADHD until he was almost 6 and diagnosed with Autism when he was 7.
It could be ADHD, but it could also be many, many other things. If you are concerned, talk to someone who can do an evaluation.