We've been here - and we ended up with a little girl with OCD - she couldn't touch anything to do with school, so no hw, her uniform had to be taken off straight away when she got home, wash, wash wash. Couldn't sit in the car where she had with her uniform on, couldn't touch door handles she had touched with her uniform on. She felt completely out of control with her life, and she was. She was told she HAD to go to school, but it turns out she didn't. We took her out for 6 months, and in that time we went knocking on doors and she has been diagnosed as having Asperger's Syndrome, and has been referred for counselling. She is back at school, although would much prefer to be back home, and seems a bit happier because the staff are now aware of the help she needs.
If you, or your DP were this unhappy at work, you would change jobs or perhaps leave. Go to your GP, your child needs help now. He might sign him off sick with stress, and you will get help with tutoring at home if this happens.
As a minimum ditch the hw, and tell the school why. He's unhappy and it is THEIR responsibility to look after him while he is on the premises. Your poor wee man, listen to him. Tell the school how unhappy you are. Do you have a local parent partnership service? Get in touch.
My solution would be to take him out of school, but I know that is frightening, so get in touch with Education Otherwise and chat to them. Your priority is your son - his physical and mental health. He can't look after himself, so you have to do it for him. Good luck.