Next year my partner's passport expires in October 2018. His Extension of Stay will be due in May 2018. Will IMM give an Ex of Stay with less than 6 months in a passport?

Yes, but the extension will only be until October 18 when the passport expires. It would be better to get a new passport before applying for the extension.

A extension of stay can be issued with less than 6 months passport validity but the extension will only be done to the date the passport expires. Then a new extension would have to be applied for at the end of the shortened extension after getting a new passport.

Best to get a new passport before applying for the extension.

Thanks guys.