I think you need anything that gets him talking and describing first, as a warm up and also to increase his confidence in using words and sentences.
I agree with Ros Wilson (who is behind the Big Talk) "if the child can't say it, the child can't write it." - so although it is a "writing" session, I would not be afraid of having a lot of talk before the writing.
e.g. warm-up games where you have several similar objects (e.g. ten green leaves, all different) - he has to describe them so you guess which one he means; games where you discuss the differences / similarities between things? Can be crazy - e.g. a banana and a phone (you can hold them both in your hand, you could have them both in Africa, ...)
Then work on describing things with the five senses and who/what/where/when/how/why. Maybe a different picture or object each week - or make it fun, take in actual objects e.g. from your kids dress-up box.
Take a boring sentence - The man ran out of the building and turn it into several different stories just by changing the nouns. The firefighter ran out of the palace, the robber ran out of the bank, the teacher ran out of the school. How many can he create? What happened next? How much more exciting would adverbs and adjectives make it?
Story mountains are very useful, am sure you would find some on TES. So he plans his beginning-middle-end or conflict-climax-resolution before he starts to write.
I personally think there is no harm in reading TO children who are behind. It means they hear correct English modelled and gives you something to talk about. If you look at the Scholastic study guide for Millions of Cats (a wonderful book), you will get the kind of idea: www.scholastic.com/browse/collateral.jsp?id=36051 This could then be turned into a piece of writing.