What Causes Numbness In Feet After Back Injury?

My dad broke his back last week Dr said is was his L4 I believe, they opted not to do surgery yet since he was able to stand while using a brace, he was sent home with the brace and told to stay in bed. He said his feet went numb today, should he go back to the hospital or wait to see his Dr on Monday? I wasn't sure if that was a bad sign.