My 4 yr. Old has fever and really bad backaches what can I use on him?

Dicitis? See pediatrician rule out discitis severe back pain in child should be evaluated by a professional nothing beats a good physical exam.
Back pain. Your 4 year old needs urgent evaluation by physician to rule out kidney infection or pneumonia among other causes and provide treatment. Back pain in 4 year old is very unusual, possibility of traumatic injury can not be excluded.