Life

A per stirpes designation means that if a named beneficiary dies before the Insured dies, the children of the named beneficiary are entitled to the benefits, or the grandchildren of the named beneficiary if the children aren't alive, or the great-grandchildren of the named beneficiary if the grandchildren aren't alive, etc.

Designations using per stirpes designations are unacceptable. You may want to consider a designation like this, instead:

Hector Gonzales, my son, 100%, if living 
Otherwise to the estate of Hector Gonzales

You could then specify the per stirpes terms in your will. If Hector is not living when you die, OFEGLI will pay your estate. The estate will follow the terms of the will which include the per stirpes terms.

You can download the FEGLI Life Insurance Designation of Beneficiary form here.