Given your son's age/grade level, you and he might consider a research or model project, in which your son learns all about Jupiter and shares his learning on the science fair poster board and/or he creates a model of Jupiter.
 
Here are some articles to help you in this endeavor...
 
An overview of different types of science fair projects: http://chemistry.about.com/od/sciencefairprojects/tp/typesciproject.htm
 
Wikipedia page on Jupiter:
http://en.wikipedia.org/wiki/Jupiter
 
Solar System Science Fair Project Ideas:
http://www.education.com/reference/article/solar-system-science-fair-project-ideas/
 
Backyard Astronomy:
http://www.education.com/activity/article/Backyard_Astronomy_Kindergarten/
 
Successful Science Fair Projects:
http://www.education.com/reference/article/Ref_Successful_Science/
 
Enjoy!

I agree that a model would be the best bet for your young son.  Windows to the Universe provides a good picture and stats comparing the size of Jupiter to Earth.  Creating models of the two for side-by-side comparison would be a great first project:
 
http://www.windows.ucar.edu/tour/link=/jupiter/J_comparison.html
 
There are several excellent sites to help kids learn about the solar system.  Many of them have pictures of Jupiter taken by Hubble and unmanned probes.  Check out Astronomy for Kids (terrific animation of the solar system), At Home Astronomy (family activities), Cosmos4Kids, StarChild, and Windows to the Universe (where you can choose beginner explanations of the planets).
 
These sites are described, with links, on LearningReviews.com's section on Planets and Astronomy:
 
http://learningreviews.com/jreviews/Science-Categories/Education/Science/Planets-a-Astronomy_c129_m55/lang-en/page-1/limit-50.html

Another good backyard astronomy science project for this age is making a sundial http://www.easy-kids-science-experiments.com/astronomy-science-fair-projects.html which also helps demonstrate the concept of time.

what so wrong with Jupiter it is a good start for science