What Causes A Small Bruise On The Spine?

My 3 year old son has a small, faint bruise on his spine (lower thoracic, upper lumbar region). It is small, faint, but has been there for about 2 months. I just noticed tonight that it had not yet gone away. Could this be a result of a bone bruise? Should I be concerned? He is a typical toddler and gets many bruises from bikes, soccer, etc...all which heal very quickly.