My wife has been having a pain in her lower back since birth of our son (8month) could this be due to epidural she used before birth to relieve pain?

Not likely. The needle used to give the epidural is very skinny, & unlikely to cause any problems. Pregnancy & delivery, on the other hand, cause a number of changes that can cause back pain. Stretching excercises, such as yoga or pilates, can help.
Possible but. Unlikely, unless it is pain at the insertion site or the placement was difficult and traumatic. More likely it is muscle and bone pain due to the stress of carrying a baby and labor. Talk to your ob/gyn.