Things I've tried so far:

First I have taken into account the interests of the boys.  They enjoy sport and using technology, so when we were looking at explanation during our brain work I chose an explanation video of a basketball lay-up for the class to compare to other types of explanation.  As the boys liked to think of themselves as 'experts' in this area they were more than happy to give the good and bad points of the video and how it compared to the written explanations.  The boys were much more engaged with this.  They went on to explain something of their choice which was mostly sport oriented.

I have found that reluctant writers find it hard to face a blank page so we used graphic organisers to plan their explanations first.  This definitely helps to 'chunk' the writing and organise their ideas.

This target student used the plan well and then wrote a clear explanation about boxing.  He usually takes ages to get started and then work through to completion but he was engaged with this task.