To some extent, writing will be improved by more reading of a wide range of material. So what is his reading level like, and does he enjoy reading?
If language skills and spelling are part of the problem, then a very useful and easy to use book is reviewed in the MN Book Reviews. If you look for the section "Children's educational books and courses" you should find the Oxford Phonics Spelling Dictionary. There is a link to see sample pages from it, and it can be purchased via Amazon.
I worked over twenty years as TA and helper in primary schools. Another tip for children who are reluctant writers, is to use a tape recorder (or phone with recording facilities) and let him DICTATE his text first. He can then handwrite it or type in on the computer. I had Yr2 SEN boys who could hand-write virtually nothing, but when they dictated their stories to me, and I typed it for them, they had plenty of ideas and enjoyed seeing their words appear of the computer.
Try to have a PURPOSE for the writing. So, keep a diary or journal during the holidays, or where you go and what you do. If he has particular interests, write about them (sport, music, etc). If he has younger siblings, or cousins, he can write little stories for them, aimed at their age level.
If he makes an effort, praise it and try not to be too critical at first. Invite him to re-read it to find his own mistakes, and see if he can use a dictionary to correct spellings. A Thesaurus is very useful to improve vocabulary, and you can also get Rhyming dictionaries if he wants to make up poems.
His reading is a bit better (3c) and as he's getting older his interest in reading is improving but he's always been a reluctant writer.
His spelling, whilst always phonetically plausible, is terrible when he's free writing. During spelling tests and if you actually slow him down and get him to look at each word, he's great. The same words back in a piece of free writing is completely wrong again.
The diary/journal is a good idea, one I'd thought of but wasn't sure if it was the right way. Do I have the skills to guide such an unstructured piece of work? I don't know. I don't want to make things worse.
I've found a kids' dictionary which I've given him and I'll look into a thesaurus. He has a new pencil/art set that I hope will encourage him.
I've also spoken to his teacher about possible dyslexia. Dh and many of dh's family are dyslexic and ds2 has a few traits (not glaringly so though) and will look into some cognitive (?) tests to rule it out when he goes back for yr 4.
I don't see that you need to guide him much?! Any writing that he 'has a go' at has got to be better than NOT writing at all. And if family members have difficulties, pool your resources, compare notes, and support each other.
Give the Phonics Dictionary a try, as it is a very different approach from a 'normal' dictionary, and it could just 'click' with him.
This sounds a lot like my ds (8 tomorrow) He has struggled in school but has been doing really well with a private tutor for maths. He seems to have forgotten a few phonics and hates writing. He's good at drawing, has learnt to play the piano (by listening rather than reading the notes) he's on stage 10 now for reading but school think perhaps he may be dyslexic! I'm totally confused as to how I can support him. He has finished yr 3 with a 2c so will be assessed by an Ed psyc in year 4