What Is The Life Expectancy If Diaphragm Muscles Are Paralyzed?

My son is in ICU--had surgery on two broken bones in his neck--spinal cord is fine--paralyzed from mid chest down and is on a ventilator/respirator waiting to breath on his own, but how long should this take and if the diaphragm muscles are also paralyzed what will be the outcome. He can t stay in the hospital forever.