What could be causing my 3 year old sons lower back pain?

See a pediatrician: It is not customary for 3 year old kids to complain of back pain. Hip joint infection or back infection etc can be possibilities. Make sure he does not have fevers and is not refusing to walk or having night time pain etc. He would benefit from a pediatrician or an orthopedic evaluation. Unless this was a short lived one time complaint.