My 5 year old boy had got a hairline fracture in his arm so had wore a sling for 10 days when he was 3 years old.after few months I observed that whenever he lifts his head up and down there is a clicking/popping sound.he has a problem pushing with his head forward whereas other sides he pushes easily.is it related to any muscular problem or is it common in kids .no other problems except this clicking and neck strength problem.xray was normal a year ago and no problem anything else.what should be done and any excercise to clear it.