What was the type of delivery? If normal, was it difficult or assisted with forceps etc? In difficult deliveries, rarely injuries can happen to the end of arm bone that presents as swelling in elbow.
It can be due to conditions such as septic arthritis
or fibrous dysplasia
Based on the x ray
findings and condition of the baby, your doctor will suggest further investigations needed. Please follow your doctor`s advice.
Unless the exact clinical condition and the findings on X ray and blood reports are known its difficult to say what it is.
Wish a speedy recovery for your baby.