About to to turn 4 and start school, and still having accidents! Despair!

[MrsHelenBee]

DS1 was only potty trained last summer when he turned 3, due to severely delayed walking (2y 9m), so we're not a year in yet but he's starting school in September and still isn't reliably dry and, at time, clean. After the initial challenges of getting him to understand when he needed to go, we had 2 weeks in the autumn when he kept having accidents at nursery (wet and dirty), then it was fine again.
Over winter, he was very unwell and on numerous courses of antibiotics, so another relapse to having accidents (home and nursery) wasn't a total surprise, and by spring, he was fine again.
We've been great since then, but now we're back to more of the same again. For 3 weeks, he's having at least one accident a day at nursery, usually wet, but dirty at times too.
He's a bright boy but when I ask him why he didn't go to the toilet or tell a grown up he needed to go, he just says "..because that's naughty" - yes, it is a bit naught DS1, that doesn't explain why it's happening.
I don't doubt blips are very common, but in the run-up to school, and when he's fit and well, I don't understand it, and I'm getting really frustrated. Have I done something wrong? What's going on?!! While we still have a few months, and it could all calm down again, I fear we could be here in September, and I can't expect school to be ok with accidents every single day. Argh!!!!