Bonnie Terry, M. Ed. answers this question:  “When my son tries to write a sentence he goes back to beginning and rereads before adding each new word and it is painfully slow.  What can we do to improve that?”  She explains that what is happening is the breakdown between the brain working and the hand writing the right thing.  She explains that he is probably aware that he makes mistakes, what they could do is have her write down the sentence as he says it and then he can rewrite it.  Maybe do this for every other sentence, eventually he may be confident enough to do it all on his own.  Another thing that could be happening is that the physical writing can be hard for him that he loses his place parents can work on penmanship for this.  Video is of good quality, it would help the parent who is looking for answers to this question about writing problems.