Out of interest, did your son go to nursery/pre-school prior to school? Just wondering if he’s been used to leaving you & having structured days with lots of other children or whether this is brand new to him?
Is there a reason he started in October and not September with all the other children?
If he’s not done nursery/pre-school - this will just all be a huge adjustment for him. He’s not been there long & appears to have started reception late. I think it’ll be worth asking for a proper meeting with school and explaining his behaviour fully and your concerns. They’ll hopefully put a bit more in place to help him feel happy & look forward to school. Small things like a support worker or even the teacher looking out for him a bit more, encouraging friendships and speaking to your son to ask if how his day is going/has been so they can address concerns. School should work with parents to settle children but having been a teacher, parents staying often makes things worse unfortunately as eventually, you have to leave!
He may also be feeling like you’ve sent him to school because of new baby’s imminent arrival & feel rejected.
After school, maybe turn up with a nice treat he likes, a favourite biscuit or something and say well done for having a fun day at school! Talk to him about his day, what was his favourite bit, favourite person to play with etc. When he starts acting up, ask him what upsets him so much and try to figure out his concerns. You could introduce a reward chart at home for getting through the evening without tears - a sticker for eating dinner nicely, having bath, brushing teeth etc and I’ve gets all the stickers at the end of the week, a special treat!
I’m sure he’ll adjust soon, it’s all very new to him. Not easy though, I hope things get better really soon!