Tax help for military filers

Yes, the entire amount he was paid while on active duty is the amount to type in the box - those were his active duty wages. 

If you have his last LES from active duty, you will see his Active Duty pay listed as "Wage YTD" (see actual LES example, below).

If all you have is a W-2 that includes active and reserve pay, you will have to find out what his gross reserve pay was after leaving A/D. His reserve unit can provide this information (if he asks the right person).