My 15 month old is constantly falling and bumping his head pretty hard everytime. He just gets way ahead himself. Should I be concerned with head injury?

Maybe. Toddlers fall and bump their heads all the time. Most of the time it just helps them learn not to fall and bump their heads. If a child does it excessively though, or is doing it repeatedly with enough force to cause significant injury (more than just bump, cry, then go back to playing), then an evaluation of gross motor skills and/or neurologic development might be warranted from your doctor.