Son had an MRI done on his brain. It came back "normal", what does that mean? Does it rule anything out? If he had tourettes would the MRI indicate?

Good. An MRI of brain shows its anatomy - that's it. So tourette's would usually show nothing wrong on a single MRI test. Research has shown that some tourette's patients have a swollen thalamus. But this can only be demonstrated with serial mri's. See my health guides on tourette's for links to the studies.
Brain MRI normal. Signifies the structures of the brain appear normal, i.e. No obvious tumors, brain shrinkage, bleeding, tangles of blood vessels (a-v malformations). Eeg's & pet scans record functional abnormalities. Sometimes a pet-mri is performed to localize areas of abnormal function. Your doc can discuss the details of the MRI report & their significance.