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.