Been there done that.
Finally got my DS16 a 504 last year because he has very similar problems with writing. Sixth grade was very difficult and I attribute some of his behavior to immaturity and puberty. Writing assignments like you describe were very very hard for him and still sometimes stymy him. In DS's 6th grade gifted class they had to write one page on 3 prompts weekly. (First draft hand written) It was free choice from a about 10-12 different prompts each week. It shouldn't have been difficult the teacher was just trying to get them writing from a prompt. This piece of homework either took forever or caused a huge anxiety attack and meltdown. Part the way through the year the teacher told us he only needed to do one a week. I'm fairly sure DS just stopped turning them in at all because I decided his mental health was more important and stopped supervising that assignment. It's wasn't going to keep him from moving to 7th grade.
What have I done? Well things got bad enough that year that I took DS to a psychologist. In addition I put him in social skills class as he was also having major social problems that year. He was having some meltdowns in class & the other kids didn't take kindly to this. Junior high things got better and by 8th grade he had no helped and was getting A's on English & SS assignments. He went downhill again in freshman year of H.S. Some of the variability is I think due to teacher fit and others curriculum. At that time I finally got DS tested and set up a 504 for him beginning of last year. Turns out DS while gifted, has low working memory and processing speed and looking back on it it's been tripping him up for years. He currently still see's a psychologist for his anxiety/depression/motivation issues. I have also tried to get him to work with a writing specialist but I have had bad luck finding someone who worked out in the long run.
Writing that is personal or wants him to tell about his "favorite" situation is still very hard for him. It's frustrating because when it's a topic he is comfortable writing about he writes very well, quickly and effortlessly. But this problem has kept him out of the top level English classes at the high school and school keeps wanting to put him in remedial writing classes which are a bore. Honestly it's a battle and I've tried various things over the years. Many times re-framing the question/prompt works wonders. One of the line items in his 504 is that he can request an alternative prompt. Although I'm trying to get him to do that by himself.
Maturity and confidence have also helped a lot. He hasn't had fits about this since 7th grade but he still has times where he just doesn't write anything.
Good Luck. Your son is NOT alone in this. One thing I've learned is PICK YOUR BATTLES.
