My 7 yr old son has a weird gait when he runs, his legs don't bend that much and it looks like his hips/pelvis doesn't move at all, should I be worried?

I would investigate. Talk to your pediatrician. Let them evaluate your child after hearing your concerns. It may be normal, especially if he has always had that gait. It may be a muscle weakness issue that could require physical therapy. It might merit an x-ray and/or an orthopedic evaluation to rule out infection, anatomic problems or trauma. I would definitely look into it soon.
Yes. Have him evaluated by a non-surgical sports medicine physician. He may have congenital hip problems or other issues.