With regards to reading/writing, it will depend on the school and teacher, but typically you will get some children who can confidently write the alphabet, wimple CVC words that they know, whereas others will start and won't be able to write a recognisable letter. During the year they will work on letter formation and letter recognition, but they might not work on reading yet- I've been in schools where they don't "teach" reading until KS1, others where they work on reading in literacy every day. But your LO won't be the only one who won't know how to read or write (if they can't at the moment).
I imagine they will be expected to get themselves dressed for P.E, but the teacher and T.A will help them if they have their plimpsoles on the wrong feet or get their head stuck in the arm of their t-shirt! During P.E lessons in the early days I'm sure the teacher will spend time modelling getting ready- reminding the children what they need to do (take your clothes off but leave your pants and vest on, put your clothes on your chair, take your P.E kit out and put it on, you should have a t shirt and shorts...).
I'm not sure about school dinners, I think they just get a tray and will be guided to the serving area and the dinner ladies will ask them what they want and help them to choose, and then they'll be guided back to the table. In my experience (I'm a teacher) the teacher takes the children to the hall and hands them over to a meal time assistant who will get them settled onto their table, help them with their lunchbox and open packages they can't do, and send the school dinner children up to get their food.
Does your LO go to preschool? If so it isn't such a change from that- yes their day is more structured,but they will still be looked after by the teacher and s/he will make sure that they know what to do.
My DS is a summer baby (early July though) and I worry about him starting school- he still seems so little to me! But then I remember how much of a Reception teachers job is nurturing and caring for the children and I know that he'll be OK. That said, delaying until next September is definitely a viable option and it is something I would consider (if my LO was a late August baby) if I felt he wasn't emotionally mature enough for a more structured school day. My friends niece is an end of August baby and they held her back a year because she struggled at nursery- she got a bit lost and upset and didn't handle structured activities well- any time they had a learning focused activity she would cry or get stressed apparently. She is now going to start school this September (she'll just be 5) and is in a much better place, more confident and looking forward to a new challenge.