Some children hate art, some PE , some maths and some writing and as jalapeno says children just have to get on with it. From your post it sounds as if he is taught to do the type of things jalapeno mentions during lessons. Personally I think if a child does it in their best handwriting first time then they won't have to re write it later ...which is pointless work.
I have one of these year 3 boys too, he doesn't hate school but hates literacy and the constant planning and re-writing. He doesn't hate school at all but I fear this will happen eventually.
I have no advice I'm afraid because I am just as stumped but my technique has been to tell him he has to get on with it and practice a lot. I've just discovered our school has some advice for parents on encouraging skills in literacy such as choosing a noun then doing a mind-map of lots of synonyms (house, mansion, shack etc.) structuring and organising the text (which is what my DS struggles with) so looking at bullet points, mind-maps, subheadings and stuff.