In plastics molecular rotation, vibration, or movement transfers heat. Things become more complex when you try to explain why one metal transfers heat faster than another. It's based upon differing numbers of available free electrons for the different metals. Such a discussion would easily go over your son's head and is far more than a 7th grade science project should involve. Have your son find out what he can about the atomic structure of the solids he is testing, and start from that point.
All material properties begin with the atomic structure and the properties of the atoms involved.
Good Luck, I hope this discussion wasn't too complex. I also hope these are the types of materials you are asking about. If your concern is with engineering type materials, like textiles, brick, etc. All rights reserved.