These days, I put the child's needs first. Stitch in time saves nine, if he's developmentally ready.
If he's distracted in class, had hassle in the past, and there isn't an effective plan in school again this year, he might love the opportunity for regular individual attention to his writer's block, particularly since he is so comfortable in other areas. He might also need to recognise he needs to take some responsibility for this too and address the problem, maybe see writing as a useful skill.
Hats off to mrz, who is so committed to offer individual attention outside normal hours, but in all fairness we have to accept what the child needs is not always available at school. Sometimes it is, particularly when parent and teacher and TAs share a common plan and support each other.
Mine are secondary school now, and looking back, any hassle at primary was time wasted. I appreciate the blessed, overworked, underthanked teachers at school and fill the gaps elsewhere, esp to pass exams.
Assuming, as kayah says, parents would not hesitate to eg pay for extracurricular skills - footie, guitar, judo? If it's not happening at school, I'd be shopping around for help.
Wish the little chap the very best of progress, and you some peace of mind!