No back pay. His pay should have continued to be deposited as if he was still reporting for duty.
Also I think this is a good thing he is "returning" This means the formal investigation can now begin since he as been cleared by Doctors.
Once placed on POW status, a single soldier with no dependents will usually have their pay sent to an interest bearing account along with 50% per diem. That money will be made available to them upon their return or given to the surviving family after the soldier is returned home or presumed dead.