My ds1 (5 and in Yr1) also has trouble with writing. His Reception teacher explained that at that stage it was hard to tell whether it was because of his SN, his age or because he is a boy and boys tend to find writing more difficult.
He's now in Yr1 and still finds it hard to write things down. He can form the letters and will even verbally spell out the words but finds it hard to put anything on paper. His teacher has explained that she needs to have something on paper because the staff have to collect evidence of work throughout the year but has taken a very different approach to your ds' teacher. When the other children are given jumbled up sentences to put in order and copy, ds1 is allowed to just glue the words on to his paper rather than write them out. It takes the pressure off him, gives him a chance to prove that he really can do the exercise, and also gives the teacher the evidence she needs. If he needs motivation to write, eg in spelling tests, he is allowed to go and sit with the teaching assistant and sh gives him the extra time and encouragement he needs to get the words on to paper. Threatening to send him elsewhere by himself would have no effect on him whatsoever.
It sounds like it could be worth speaking to the SENCO and maybe even asking to see the Ed.Psych.