Has your GS had any additional support following his mother’s death? It could be that he is struggling to identify feelings of loss or fear and it’s affecting other areas?
Another suggestion is to remove emphasis on writing and of labelling it as such - so maybe not saying right it’s time to do your language work (or whatever it’s normally called).
My eldest nephew is autistic and has a pathological fear of handwriting things due to a well meaning TA trying to help him with his handwriting as a leftie a few years back. The experience for him left him lacking confidence and like he was stupid or a failure (this was due to the TA persevering with him and trying a practice makes perfect approach - the was no harsh words or anything), but it took years to discover that trigger as he struggled to identify and name his feelings, instead melting down or disengaging.
His support worker now has shifted the emphasis from handwriting to creative reading and writing on a subject which he engages with - so for my DN that’s formula one. It’s still a daily struggle, but he at least attempts to engage and will read and do creative writing on his tablet. They have managed to expand this out and include other subjects he will engage with. He has a mainstream reading and spelling capability, his handwriting is still not great, but in a technological age with his challenges this different approach has been wonderful for him. As far as I know this is blended into his day as to announce that subject would induce panic.
I’m not sure how helpful any of this was for you, but hopefully you make some progress with him and he starts to build his confidence, wishing you all the best.