bebumba DS1 had a similar experience - it was his Y6 teacher who took the time to focus on it with him, saw he was forming letters incorrectly and then spent months providing handwriting exercises for him. He's dyspraxic (though nowhere near as severe as many) and her view was he needed to use 'muscle memory' to relearn his letters. It really, really helped, though his handwriting is still not brilliant. The difference between mine (I'm also dyspraxic) and his at the same age is striking, I don't know what my teachers did with me in the earlier years but it worked.
He's in Y7 now and his school definitely do care about handwriting and SPAG. His English teacher advised him that he had lost a whole grade for the first term's work because of his simple grammatical errors like forgetting Capital letters, commas, misspellings where he really knew better and also his handwriting was illegible sometimes. Sadly using a laptop seems to be even worse for him. I know they'll work hard with him to overcome it though, as they don't want to bring down their '99% of boys get all grades A*-B at GCSE' madness. Well, either that or they'll ask him to leave...