A I think you need to try a variety of approaches that highlight the link between writing and reading. First, scribe for him - let him tell you what he wants to say and write down his words. This creates a piece of writing that is his, but without the burden of writing it.
Children also enjoy seeing their words typed on screen. You can print their sentences and stick them into simple books that they can illustrate. "I like..." or "I wish..." books or ones modelled on favourite stories work well. These can then be used for reading.
A computer program with a word bank and picture or speech facility such as Clicker or Textease is a wonderful resource.
You will need to prepare the word bank, but the child then clicks on the words needed to quickly create a text which, when printed out, provides a satisfying outcome.
Again, it is best to create texts with a lot of repetition to build familiarity with a group of words.
You can also record children's words on computer using the built-in Windows recorder and a microphone. Then, using a presentation programme, such as PowerPoint, create your own talking book.
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Neither writer can enter into correspondence with readers.