What mum teacher said with regard to quantity. Some children write teams but it starts off well and the just ends up being unstructured waffle...
They will be looking at many things. Your son will need to demonstrate different techniques within his writing. He will need to write a certain amount to be able to demonstrate the variety needed.
I'd say that he doesn't need to be able to write a whole page and a half yet though. I'd suggest 3/4 of a page of well written work is sufficient for end of Year 1.
However, having a clear structure is important and he should be able do a beginning, middle and end.
Concentrate on extending those sentences, by adding in exciting vocabulary, and using connectives to push his ideas 'further'.
If he loves reading, get him thinking about the sentences he reads in depth, why is it written that way, why are those words chosen. Get him substituting words in stories he reads for more exciting ones. I find that this is the best way of teaching 'excitingn vocabulary'. They have to have really experienced it, to understand how and why to use it.
Spelling and grammar obviously important. By the time of the 7+ I'd expect him to e very secure in full stops and capitals, proficient with exclamation marks and question marks and make good attempts at using speech marks and commas.
I hope that all makes sense.
Push for handwriting support though as first impressions count! That's something you could work on quite easily at home. But a good prep should offer support for those struggling.
Happy to answer any further questions - I'm a yr2 teacher in a london prep.