Hi Tina,
I wasn't able to follow all the details of your post but I get the gist, which is this. Your son is very unhappy at school, has been bullied, has self-harmed, and has himself been acting violently. You don't feel that the school takes your son's claims of being bullied seriously. You would much rather not send him to school at the moment. Is that about right?
Under the circumstances I think you should definitely keep your son off school in the short term at least. I wouldn't send him back at all until you think he will be safe and happy there.
You are unlikely to get prosecuted for having him off for just a few days. Some LAs are more gung-ho than others, but even if your is, it will be something like a £60 fine per parent for the entire absence (not per day). If you think school could work out for him, get the GP to sign him off school with stress for a few weeks. That will buy you some time to figure out what to do. Did you say your GP is away at the moment? If so, use whatever locum or emergency out-of-hours service is available to you. This is an emergency. See how quickly they can get him referred to CAMHS.
If you are quite sure that school is wrong for him at this time then a good solution could be home education. To do this, if you live in England or Wales and he isn't at a special school, you just have to send a properly-worded letter to the school. There is no notice period and you don't need anyone's permission. He never has to return at all. Then you could come back on here for a chat about how to go forward with home education. You can plan the details as you go along; you don't have to have it all mapped out before you start.
Once he's being home educated, you can change your mind and return him to school whenever you want. However, he might not get a place at the same school if it is full, and might have to go elsewhere.
Here are sample letters for deregistration from school in order to home educate: www.educationotherwise.net/index.php?option=com_content&view=article&id=77&Itemid=69