If handwriting is an issue, definitely try to get a laptop in place, as well as any other access arrangements the school are willing to support- what's available at this stage will vary school to school. If they mean his writing is not coherent, that's a much bigger issue and may affect a wide range of exams.
I think the best person to speak to is his teacher, though- they will know him, and know if it was a one off bad exam, or if there are deeper problems here.
I know a poor grade at English seems like a disaster at this stage, but if you look at local sixth form colleges, there may well be options for him to start a level 3 course and resit English alongside, providing he gets enough grade 4/5 GCSEs overall. This might not be A-levels, but could still lead to uni etc.
He may actually be better off getting a 3 this year, and thus being eligible for a free resit and teaching, than scraping a 4, and missing uni entry requirements (assuming that is the end goal) but not being able to access teaching for a resit.