I've posted before about trying to find creative ideas for helping students with improvisations. Here is another idea that's a bit outside of the box.
Lakeshore Learning has some great free printables available on their website. One of my favorite categories to use are the writing prompts. They issue a calendar for each month with a writing prompt for each day of the month.
While these prompts are of course designed to be used for writing exercises, I think they make great ideas for improvisation. You could either have your student pick one idea from the month, or the week, or simply have him use the topic for the day on which his lesson falls.
As with any improvisation exercise, have the student describe his answer. Then help him to think of ways to express his idea (or ideas) musically.