The Workbook_Open procedure is called immediately when the workbook is opened. At that point, Ontimer_s does not have a value yet, so you must specify it in Workbook_Open:
Private Sub Workbook_Open()
Ontimer_s = Now + TimeSerial(0, 1, 0)
Application.OnTime Ontimer_s, "SaveBook"
End sub
The SaveBook procedure should not have any arguments. And do you really want to use Save As each time? Why not simply save the workbook.
Also, I wouldn't use 1 second as interval. It would make editing the workbook virtually impossible.
Public Sub SaveBook()
ThisWorkbook.Save
Ontimer_s = Now() + TimeSerial(0, 1, 0)
Application.OnTime Ontimer_s, "SaveBook"
End Sub