My DS is yet to start school, but as someone who left school when they were 13, and started this when I was 10 I can try give help as best I can.
With me I suffered really bad anxiety and panic attacks as a child, started as I thought someone was picking on me in assembly (in reflection it was a comment made, but I made it worse for myself and refused to go into assembly from there, into not going at all).
Sounds like this has spiraled a tiny bit, and the more it spirals, the more it gets out of control (or it did with me anyway, he could very well right it himself).
Listen to him, tell him to talk to you and reassurance is key (my parents didn't do this and treated me like a naughty child). Have you bought him stationary ect yet? Maybe make a big thing of taking him to pick a new bag, new pens ect.
Hell probably find, the worst is going in but once he's there he relaxes more into it. Is there anyway you can have a chat with them and get teacher to check before he goes to lunch he's doing OK? But again it's making a big thing of it, if kids notice somethings going on they home in on it.
Maybe suggest as well end of the week he can have a friend over for tea? Give him something to look forward to.
Good luck, having been through this I feel for him not wanting to go, but make sure you really do listen to him and deal with it positively xx