If it's hanging limp, there is usually one of two causes. He either suffered a trauma (jumping, running, being stepped on) or he has a back or spinal injury.
You need to go ahead and give your vet a call today if it hanging limp. This indicates that he has little or no feeling and this is very serious.
Most vets have an after hours service so go ahead and give them a call.
Let me know if you still have questions.