My son was similiar, so I copied a page but without adding punctutaion or paragraphs from a hilarious scene in Harry Potter (white ferret).
I then asked him to read it, with and without punctuation. This made him realise how important punctuation is, and motvated him to make more of an effort, which he did. Later I did a similar exercise, but this time without all the adjectives and adverbs, from a page in a Dr Who book he had enjoyed; worked a treat. Still have the odd lapses, but generally much better, and stories are getting longer too.
Also uses Thesaurus alot more.
It might also be worth going over when and where to use puntuation, paragraphs etc.