Hello and welcome to HeathcareMagic!
Is your son experiencing pain during an activity involving the ankle or is the pain present throughout even while the ankle is at rest? Bruise
occurs when the tiny blood vessels around the ankle burst open and bleed, and thus cause a bluish red discoloration. A bruise alone, is not not a cause of worry, since it would settle down within a few days, but moderate/severe pain
at the ankle joint needs further evaluation. So ruling out a fracture
at the ankle is important. It is necessary to visit your orthopaedician at the earliest, who would take a x-ray and decide based on the findings seen in the x-ray.
best wishes and regards.