May Maglock wrote: He's a super reader but not as much a writer, so this is also to encourage more writing.
This is not what you're asking for - my 10 year old has morphed into a 30 year old engineer who creates board games as a hobby.
However, as a 10 year old, he too, struggled with writing. We were working through a book called, "Grammar with a Giggle" and part of the daily assignment was to take the "word of the day" which were slightly uncommon English words like "verdant" green, and write a sentence that used the word in a reasonable context. Our house rule said that the sentence had to be a minimum of 13 words, but I'm not sure where that came from (20 years ago...) The idea was to make sure
he was challenged to write one complex sentence every day. It worked wonders. Eventually, out of choice, he started linking the days together with an overall creative theme. It got him over the fear of "writing a story".
There is no reason you couldn't just make your own list of English words to be the target words, or find a book that's at your son's limit or above for vocabulary and choose words out of that book. I used the Grammar book because we were using it to help him learn proof-reading and to hopefully help with some of his dyslexia issues, but there are plenty of ways to generate a suitable list of words.
Good luck with finding a pen pal.