Don't worry! DS1 sounds very much like your DS. His SATs for Maths and Science were high 5's and he was G&T for Maths, however for English he got 4a (4b for writing and 5b for reading - I was concerned so I asked for the breakdown). I was worried that his writing wasn't up to scratch but his primary school teachers said not to worry as they were surprised that a boy who read so widely and had excellent vocabulary could get a relatively low score for the writing part of the SATs paper.
He is at GS and I think being in an all boys environment has helped enormously - writing is no longer seen as girly and the teachers tune in to the way boys' minds work and pick subjects that interest them. At his first Y7 parents evening the English teacher sang his praises and he has gone from strength to strength (he's now Y9). He still dislikes writing and actual pen and paper writing is tortuous but a lot of his homework and extended tasks can be typed so he does that. The standard of his written work is very impressive (not that I'm biased
If your DS "makes the grade" then his written English should not be an issue. Many boys hate writing and are significantly stronger at Maths and Science - the schools know this and tailor the learning accordingly.