Hi MrsHeffley:
Your DS sounds a lot like my left-handed DD now in Y4 - who received a 1a in KS1 SATs but now is greatly improved. We opted for writing on the sly.
-Postcards: have DC send postcards from every place they visit (even if only a day trip to a local historic site). Send them to favourite teachers, friends, grandparents, etc...
-Competitions: have DC enter competition - but use the postal address & snail mail.
-Father Christmas: insist that a letter for Father Christmas is drafted each year.
- Birthday cards - have your DC make hand-made cards for friends birthdays or if store bought have them write something more than just their name. DD started with Hope you have a nice day and now expands to things like Mum and I found this when we went down to Grandma's - I've been waiting ages to give this to you. I hope you like it!
-Thank you cards - yes it's very old fashioned, but insist that your DC sends thank you cards for presents, treats, nice days out.
Now this may be more a girl thing than a boy thing - but my girls are constantly trying out new and interesting ways of writing (very into swirls and curls) - so I've let them chose a little hard-bound notebook and pen from an office supply shop. They loved chosing if for themselves.
We also found that using crayolo twistable pencils and more detailed colouring books also helped with fine motor skills (e.g. geometric designs - see www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=geometric+colouring+books&x=0&y=0)
Finally we also found a left-handed pen helped a lot (we use stabilo left-handed pens - can get these from WH Smith & other major office supply chains).
Our DD really complained about writing - but it turns out she does very little actual writing at all at school. So with school work - I'm afraid I opted for just doing slightly more.
So with spellings: encouraging DD to write out sentences using words. If that's already required encouraging DD to research with on-line dictionary the origin and age of the word. Most kids respond well to words being from the Vikings.
With reading diary: Our school provided some suggestions of what could be done and only requires one entry a week (which can be as short as 'I read Chapter 3') - so I encourage 2 entries a week. Sometimes it is just drawing characters with speech bubbles, but it's still working those muscles.
HTH