Yeah, I admit it is broad, but I found some of the comments particularly helpful. Especially in how you ask him to write. My DS really dislikes writing - it might be his least favorite thing to do after making his bed - but he will write about a science experiment..or a video game he wants to make up..pokemon (for goodness sake), etc.
Now, I think I get why his journal is always blank (well, usually it is covered with drawings) when there is no prompt, and they are supposed to "write whatever they are thinking about."
