If the teacher is saying, as a statement of fact, that your child's language skills will suffer due to problems learning at this age, then the teacher is flat wrong. More than that, I would be deeply suspicious of their teaching methods. It's not impossible that the teacher is the problem here, and your child's reaction is due to the teacher's attitude towards them. If this is the case, it's even more important to work out what the problem is.
Hand-eye coordination and other motor control skills don't always arrive on time. More practise at things like drawing which stimulate those skills might help. See for example this link about pre-writing skills. If your child is getting frustrated about not being able to control the pencil, and especially if criticism from the teacher is making them feel bad about the whole thing, then the teacher needs to change their expectations and build up the basics a bit more.
Language comprehension and use is another part of this. Have you started correcting their inevitable misuse of grammar so that the child picks up how to get past "baby speech"? When they're capable of following and expressing more complex concepts in time, positions of things, and what they want to do, then they're definitely able to use "proper" language to describe them. They should be used to you helping them to use that language. If they aren't, this could be a cause of frustration for them and for the teacher. You may need to step up on this, but the teacher also needs to back off whilst your child builds their skills. This link describes those kind of prerequisites to writing in a bit more detail.
But a further issue could simply be that the child is afraid of the teacher and this is affecting his behaviour. I would hope the teacher hasn't shouted at him, or told him off for failing to write well enough, or anything like that - but it's possible.
This isn't an easy place to be. You do need to be talking gently to him about why he's crying though. As usual for children, suggesting what it could be and asking "is that it?" can be helpful, and then showing that it's not something which worries you and it's something you can all fix. If it does come out that the teacher is what scares him, then this link has some strategies for how to work with that. As the link says, the teacher is unlikely to be doing this deliberately, so it's similarly a matter of working it out together respectfully. Your question does make me concerned though, because it doesn't sound from that like the teacher is respecting your child. Whether that's the case, or whether that's just how it sounds from you shortening the whole episode to ask the question, only you can answer.