Hi Wishihadabs:
MN has a statement on progress through sub-levels here: www.mumsnet.com/learning/assessment/progress-through-national-curriculum-levels
The thing to take away from it is that it is 1 to 2 sub-levels progress a year - so one sub-level progress isn't great but it isn't a disaster either.
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It's always difficult to know if problems with writing are about handwriting (hard to read, not fully joined up, etc....) or about the use of vocabulary/ punctuation.
If it's handwriting - I'd encourage him to do as much writing as you can during the summer and really stress neatness.
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If it's more a vocabulary/ grammar thing - it may help to learn about VCOP (VCOP is a pyramid with progressively more advanced examples of vocabulary, connectives, openers & punctuation): more info here: www.keystage2literacy.co.uk/vcop-menu.html - click the icons for info. There's a nice summary pyramid model here: <a class="break-all" href="http://www.google.co.uk/imgres?imgurl=www.staveley.cumbria.sch.uk/Images/Pyramid.jpg&imgrefurl=www.staveley.cumbria.sch.uk/BigWriting.htm&h=300&w=400&sz=13&tbnid=Gdn8qq5fk8TZ6M:&tbnh=92&tbnw=122&zoom=1&usg=__tHS9CTlrBuUBR3g8ksMi-ofw56M=&docid=i0pzuGMYfNTmPM&sa=X&ei=UNHnUZqHJKap0AXir4DwBA&ved=0CGEQ9QEwCQ&dur=2750#imgdii=Gdn8qq5fk8TZ6M%3A%3BcLRuz5zDE8eQfM%3BGdn8qq5fk8TZ6M%3A" rel="nofollow" target="_blank">www.google.co.uk/imgres?imgurl=www.staveley.cumbria.sch.uk/Images/Pyramid.jpg&imgrefurl=www.staveley.cumbria.sch.uk/BigWriting.htm&h=300&w=400&sz=13&tbnid=Gdn8qq5fk8TZ6M:&tbnh=92&tbnw=122&zoom=1&usg=__tHS9CTlrBuUBR3g8ksMi-ofw56M=&docid=i0pzuGMYfNTmPM&sa=X&ei=UNHnUZqHJKap0AXir4DwBA&ved=0CGEQ9QEwCQ&dur=2750#imgdii=Gdn8qq5fk8TZ6M%3A%3BcLRuz5zDE8eQfM%3BGdn8qq5fk8TZ6M%3A
What I've found is that by knowing where my DDs are working (what punctuation marks/ vocabulary they regularly use and which they don't) and what is 'next on the pyramid' - I can gradually encourage them to use the next step VCOP things to build skills. It's a very gradual thing - but it has helped for us. With DD1 - she has a tendency to always use because, but forget there is although, instead of, as a result of, etc...
You can sneak this kind of thing in their writing: for example if they have to write out sentences for spelling, encourage them to use an interesting opener: Unexpectedly, the knowledgeable (that was the spelling word) teacher struggled to answer the question and all the children laughed.
Encourage accurate punctuation when you see them writing (letters, e-mails, etc... - double checking and making corrections as they do it does help).
Obviously without knowing your DC, it's hard to say this is the problem - but hopefully this helps.