Zu viel auf einmal
For a lot of people that's the problem they have with writing: I have to be creative, I have to write a lot, I have to spell words correctly, I have to use certain vocabulary words, I have to write fast... It's too much. For that reason I'd have him break down the things he has to improve and have him work on them separately. Vary which skill he's focusing on (fluency, accuracy, expression, vocabulary) and gradually they'll come together.
One suggestion would be to have him do a number of different kinds of writing. Sometimes have him write as much as he can in two or three minutes without worrying about correctness: just try to write a lot. For example, write as much as you can about your favorite computer game, write as much as you can about yesterday's soccer game, imagine a group of students your age is coming to visit your town and brainstorm a list of everything they might want to do. He wouldn't even always have to write complete sentences. Sometimes it could just be words. You could make a game of it: each of you could pick a movie you've seen and then compete to see who can write the most words that have something to do with it. Not sentences, just individual words. When you are done, compare your lists and explain why you put certain words on the list. Limiting the amount of time he has to write to two or three minutes forces him to improve his fluency, but in order to foster that, you should remove the pressure of accuracy.
Other times have him write longer pieces, paying attention to just two or three things. For example, write one page explaining why a character acted a certain way and use "when" and "because" to create complex sentences. Or he could write a review of something (a book, a play, a concert) and focus on writing three paragraphs, using certain transitions to connect the paragraphs. Have him save what he writes and then once in awhile ask him to go back, pick something he liked and improve it by revising it (correcting spelling and punctuation or clarifying things by expanding certain parts).
To get him to pay attention to how he writes something, take some of the pressure of content off him. You could ask him to write the same thing for different audiences. For example, if he has a pet, have him write instructions for caring for his dog while he's on vacation. One time he writes them for a seven-year old neighbor who's just learning to read. Then he writes them for a friend his age. Then he rewrites them for an unknown adult who happens to be afraid of dogs.
To practice writing more and providing detail, you could also give him the stub of a story and ask him to make it interesting by filling in the gaps and adding as much detail as he can. The stub could be something like the following: I woke up. My mother told me something. I ate breakfast, then walked to school. On the way I saw a guy. During the break I played table tennis with my friend. After school we stopped at the park. I lost something. Then I went home.
If he's focusing on adding detail, don't have him worry about getting the spelling or grammar right. Once he's written a story, he could go back another day and focus on finding and fixing errors.
As for texting or chat rooms, those aren't bad per se. The only problem would be if he's mixing chat- or text-speak into school writing. If that's a problem, then you could have him play around with the different styles. Have him write a text exchange in which he tells a friend about something that happened. Then have him rewrite the conversation for an English-speaking adult who doesn't know any text gimmicks.
Just some ideas.