You could restrict by the column index: if your button is on the fifth column, for example, it's index is 4. So, instead of:
if (e.RowIndex >= 0 && ((DataGridView)sender).Columns[e.ColumnIndex].GetType() == typeof(DataGridViewButtonColumn))
you can write:
if (e.RowIndex >= 0 && e.ColumnIndex == 4)
With the inconvenient that you have to change your code whenever you modify the index of the column.