In addition to the resource previously suggested, I would also recommend these links below, which include an information center on "Helping Your Child With Science Fair Projects," as well as an article on the scientific method.
Good luck - I hope the project goes well and you both learn a lot!
I would first ask your son, what does he think he should do. We all want to help and support our kids but sometimes we need to step back and let them figure it out for themselves.
I assume the science teacher has given the students some direction on how to use the scientific method, a control, ect. Let him start the project, because if you start the project from the beginning he may let you do the whole thing.
I suggest you do your own research and guide him when he needs the help. Whether he gets an A or B, it will be his A or B.