My 9-year-old Son Fell On Ice 2 Days Ago And

My 9-year-old son fell on ice 2 days ago and injured his upper arm - about midway between the shoulder and elbow. There is a bruise, and it is quite sensitive to any pressure. I don't notice any significant swelling. How can I tell if it is just a sprain, or if I should bring him in for an x-ray to see if it is fractured? Thank you.