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

well you can do like me im in the 11th grade and i did a science project heres some information:


Jupiter is the fifth planet from the Sun and the largest planet in the solar system. Jupiter is a giant gas planet which is made up of about helium  and hydrogen. Jupiter does not have a solid surface because of the gas. The swirls we see when looking at Jupiter is the top of clouds in high atmospher.Jupiter has sixty-three moons.  To the ancient Greeks, Jupiter was known as Zeus, which was the ruler of the Mount Olympus and Greek Gods.

 Jupiter has alot of speed that goes more than 400mph! The bands are all different chemicals and temperatures. The bands give it a color that makes each different from the other bands. The light bands are called "zones" and the dark bands are called "belts"
 Jupiters most outstanding and interesting feature is called "The Great Red Spot." It is big enough to hold two Earths!
 Jupiter has dark and small rings that are like Saturn's.
 When its night outside, you can see Jupiter because it is the brightest star in the sky. If you have a telescope you can also see the bands and the Great Red Spot.