most children of this age don't have automaticity for handwriting, so that wouldn't distinguish him from age-peers.
It's worth keeping in mind here that his teacher may simply have unrealistic expectations for a boy this age. There's been more than a few discussions on this board of kids getting labelled ADHD or LD for very normal behaviour in 6-year old boys.
Why has he not figured it out by now with practice? Is there something else we should be doing to help him?
However, it's also worth noting that not automating a task, despite practice, is pretty much the definition of an LD, so this is an important question.
It comes down to whether what he is being asked to do is reasonable and developmentally-appropriate for his age, and for a boy. Is he really way behind, or just slightly lagging, or actually quite normal? The fact that he spent all last summer practicing writing as a 5 year-old raises a lot of red flags to me about his school's expectations. As do several other things you note, such as a inability to "stay in his seat" - in kindergarten! - and his teacher's belief that an advanced reader should somehow magically also have equally advanced fine-motor control.
All that to say, definitely don't ignore your own instincts if something feels wrong. Vision issues have undoubtedly made everything much harder for him, and it's really important to find and fix these things as early as we can. But do take a long hard look at his environment, before concluding the problem is the child.
