My 7th grade boy sounds very similar. We think he has some undiagnosed learning disability/ies but in the meantime we have had him working with a writing coach, in our case Kristin Hawkinson, and it has helped as far as written expression. He still has a ways to go though! Your son is not the only boy like that.
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I highly recommend Kristen Hawkinson, M.Ed. I don't know if she is taking new students this late in the year but your son would be in a good position for the fall if you got in touch with her now. Kristen is exceptional! Her contact information is 510-526-8701, kristenhawkinson at comcast.net Nancy