I suggest you do the following:

1. Loop through the range  of 13 cells.

2. Add to a new string (e.g. strFormula) the values of all cells  that
are not blank, separated by commas. At the end of loop, strFormula
will be equal to something like:
value1,value4,value6,value7,value10,value12

3. Assign strFormula to 'Formula1' property of 'Validation' object of
your target cell ($H$14), which will contain dropdown list  based on
what was chosen, excluding any blanks.