My son isn't at local school yet, but I teach so I've seen some of the things students do at local schools here. Obviously so much depends on the school and the teacher but there are a few things to note.
Local schools are big on grammar. Your kid will know what's correct just by being a native speaker, but will need to learn to answer questions like `what is the past perfect form of to have?'
He'll need to learn spelling, and how good he is at that will depend to an extent on how much reads.
He will probably have to do utterly pointless 'dictations' in which he has to memorise a passage and then write it out exactly as the original.
Also, sometimes things that we say naturally would be marked wrong (e.g. 'he was later arrested by the police.', 'we will get a new teacher next year, I hope they will be nice')
I've also seen some shocking examples of 'good' english from even the top schools (sometimes even the exam papers are wrong!) So it's not as straight forward as you might think. But a good teacher/school will make all the difference.