So he retired at 58 and would have recovered his cost in 260 months -- by 2016.  Usually, though, when I see a blank for "taxable amount," there is some special situation, like disability retirement or a survivor pension.  Helps to know when he started his federal career, to figure how much he would have paid in after-tax contributions.  

Rule of thumb:  If he has a pension file dating back to when he retired, take a look.  Otherwise, it's all taxable.  And Pub 721 is a good source.