The only thing I would add, is kids do go through a waffley, no punctuation, no paragraphs, terrible spelling, no structure phase as they realise they can form fun and exciting ideas with words, and write pages of nonsense about invading alien mice v spiders.
Often when the ideas and the structure collide the sub level bounces up in two's.
I am fairly sure your child's teachers will be helping him with his writing, particularly as you have reports every term, they are obviously on the ball. Son number three was a 3a at the end of year 4, at the start of year 5 his teacher said, it is purely because he won't form paragraphs, so we cannot tick that box, she gave him some personal time, and I told him to do it (he is a prolific reader) and by the spring parents evening he was a 4b, his last assessment piece was marked a 4a.
Also....just a little nag, but hey, his other progress is blooming fab....big up that boy whatever plans you have for his future education.