My ds1 struggled with writing all through primary and often wrote only the date in year 1 and 2. He was very able at maths and was reading way ahead of his peers. He found writing so difficult despite having a good imagination.
Ds1(15) is now in year 11. His handwriting is still difficult to read ( doesn't like typing) but his main issue was mental blocks when writing and this can still sometimes be an issue. However at parents evening recently I was told that he is predicted to get an A in his GCSE English paper.
I tried so many things over the years.
Mind-mapping can be helpful as the ideas don't have to come in order and it takes the pressure off. I suggested this to his very supportive year 4 teacher. She used it whole class and it helped him get something down on paper.
Silly sentences: You write a sentence/ start of a sentence and fold down the paper. He writes the next bit and folds down the paper. Someone else writes the next bit. Continue until the page is finished. Then you share the silly story.
Clipboards for shopping lists, current interest and even writing out maths problems for you to solve ( model first).
Make up stories together verbally using odds and ends where one object is the main character and for e.g. a paper clip becomes the treasure. Introduce a villain, something that goes wrong a hero e.t.c. Don't worry about a name for the story until the end. I bet many famous authors can't think of their book title at the start. To develop this further use drawings or indeed the objects to turn this into a story board/ story plan.
If he can tell stories verbally type/write the story for him with his name as the author. It may give him a great sense of achievement.
I now work as a key stage one teaching assistant and find children love acting out stories using props such as dressing up clothes, puppets and silly voices. Learning should be fun.
I would not be happy at anyone stopping my child from doing maths. My ds when aged 6 would probably have refused to do anything if his teacher had used maths as a reward for finishing writing.
By the way ds still loves maths and is thinking of doing further maths as at A level.