Why not brainstorm with your son since this is his project??!
Some ideas are craftstore supplies like styrofoam balls and constructionpaper cutouts for different parts maybe attached with pins to the forms. Modelling clay?
At 7th grade your son is competent to complete his own project and generate ideas for construction. If not - he should not enter a project, perhaps...
I think it's really important to let his ideas shine through as/if you assist him. To do the project for him seems unfair to both your son and other students doesn't it?
Other people asked questions on similar topics, check out the answers they received:
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them