Have the school not held an information evening for new parents yet? Most of the questions you ask will be covered then, or in your welcome pack.
Toilets in schools are always sized appropriately for the children in that year group, don't worry. They're not going to be sitting on regular sized toilets, needing special steps or seats like they may still require at home. You say he uses a potty at home. That's great that he's toilet trained but you need to make the transition to toilet now to get him more use to it. What does he use at nursery? It's true school staff can't help with routinely wiping, unless an intimate care plan is in place, but they can guide a child to use wipes themselves if they're in a big mess. Definitely mention to them that he struggles wiping when he starts, so they know to be aware that he may need support with this. Do you receive a home visit in September at all? That's always a good time to have private chats like that.
It totally depends on his childcare setting as to whether you can still send him in August. Most pre-schools and school nurseries close during the six week summer holidays (the ones which operate in school hours) but private day nurseries are open all year. Again, you should really know this by now as, if they're closing for the summer, you'll have the last date of term written down somewhere in your correspondence from nursery. If it's a private setting, you'll need to decide when his last day will be and inform them so that you don't end up paying for sessions you don't wish to use.
It's up to you regarding his lunch. He will be entitled to universal free school meals up until the end of Year 2. Most schools offer both hot and cold options, but you do need to book these in advance. However, most schools will also allow the children to bring a packed lunch from home, if you'd prefer.
I know that starting school seems like a daunting step, especially if your child has only just turned 4 and especially if you've never done it before, but law of averages suggests roughly a third of his class will be summer born. The school will be well use to dealing with children that young. Many schools or LAs do refuse deferral unless there's a kjown, diagnosed need.
Learning through play is at the heart of the Early Years Foundation Stage, of which Reception forms the final year. Therefore, although he will be experiencing some big changes (new setting, new people etc), Reception is largely an extension of nursery. I'm sure he'll adapt to the 5 days pretty quickly. Kids are generally very adaptable and it won't be long before this is the new norm.