My son got a concussion & skull fracture a week ago and is now running a fever. Should we be worried or have him looked at?

Absolutely. With the skull fracture, and depending on location and sinus entry, serious infection can occur.
Best to be safe. With concussions & skull fractures, always the concern of infection. So a fever can be worrisome. Have your son checked out by doc to r/o infxn.