Hi, Here are some solar system science fair project ideas from NASA:
http://www.education.com/reference/article/solar-system-science-fair-project-ideas/

Here are some related science activities on Education.com as well:

Map the Solar System
http://www.education.com/activity/article/mapping_solar_system_first/

An Out of This World Solar System Project
http://www.education.com/activity/article/solar_system_project_third/

One Education.com member with twin first-graders adapted this activity to make an 'edible solar system':
http://www.education.com/activity/article/Edible_Structure/
See the comments section for a photo of the solar system structure her child made with toothpicks and marshmallows.

Review the guidelines your son's teacher has provided for the project to make sure that any of these projects above would meet the assignment. Also, here are some additional resources on Education.com to help explain the scientific method and how to assist your child with science fair projects:
http://www.education.com/topic/help-child-with-science-fair-projects/

It's important that your son choose a project that really interests him. This will help with completion of the project and in developing his long-term interest in science.

Good luck! And if your son chooses any of the activities above, do upload photos of the finished project (in the comments section on the activity page) to help other families in doing the same activity. Thanks!

ok when i was in fourth grade i had a box and i spray painted it black and then i painted my planets and i took like fishing string and i hung my planets up in my box and i got a 100 so that's what i did but u say u have a poster board well u should hot glue your planets in order on there and then buy some glow in the dark paint and brush them in the planets so when they turn out the lights it would light up and look really cool.

Try using an inverted umbrella. Remove the umbrella covering, and stick. Then you will have the skeleton of the umbrella.place umbrella upside down and where the stick was, in the middle you will place the earth, and use the exposed wire for the rest of the planets. Get a box that will cover the top of solar system and back, have it coverd in black construction paper, and use glow in the dark star stickers.

Hi there, I gave a tip about "How di I make a solor system Project?" I am so embarrassed that I suggested to put planet earth in the center, as usaual I (like every mom out there) multitask, so I am typing and writing info for two different solar projects. But the inverteted umbrella solar system really is a prize winner. Good luck. :)