I think the following should do it. As you have discovered, what you might think of as a word and what VBA thinks is a word do not necessarily coincide.
Code:
Sub Pluriel()
Dim oRng As Range
Dim lngCase As Long
Set oRng = Selection.Words(1)
lngCase = oRng.Case
oRng.MoveEndWhile Chr(32) & Chr(160), wdBackward
oRng.Collapse 0
If lngCase = 1 Then
oRng.Text = "S"
Else
oRng.Text = "s"
End If
lbl_Exit:
Set oRng = Nothing
Exit Sub
End Sub