Is he keyboarding yet? I think a lot of little gifties 'blossom' around this time when they starting getting good at keyboarding.
I think that best way to learn to write is to write, so if there is a blog or forum that centers around on of your son's interests that might be something to encourage.
If he lacks ideas, I would encourage him to write sequels to his favorite stories. We learn so much by imitation.
Brainstorming software, such as Inspiration can be helpful to some.
I'd try and incorporate 'idea generation' into everyday problem solving. Put him on the 'what's for dinner' team and encourage humor 'alt.dinner' ideas.
Good luck,
Grinity