the 1931 Canada census is not yet released so no luck there.
you may find him on a voters list, had a quick look with no luck.
Canada Passenger list
Name:Robert Mann
Gender:Male
Age:32
Birth Year:abt 1894
Birth Country:Scotland ...Stranraer
Date of Arrival:10 May 1926
Vessel:Athenia
Port of Arrival:Montreal, Quebec, Canada
Port of Departure:Glasgow, Scotland
Margaret 32 b Kilmaure
Elizabeth 10 b Glasgow
James Sloos 8 months b Aloa
Margaret Sloss 8 months b Aloa
going to Land Settlement, Winnipeg, Manitoba
borther John Sloss, 32 Fore Street , Kilmarnock
passage partially paid by ESA (don't know that term) ?Emigrant Settlement Act ??
UK Incoming Passengers
Name:Robert Mann
Birth Date:abt 1893
Age:38
Port of Departure:Montréal, Québec, Canada
Arrival Date:4 Jul 1931
Port of Arrival:Glasgow, Scotland
Ship Name:Antonia
Shipping line:Cunard Steamship Company Ltd
Official Number:145937
Robert 32
Margaret 32
Elizabeth 15
Margaret 6
James 6
Sarah 3 b Canada
Robert 2 b Canada
William 8 months b Canada
unfortunately doesn't say were they had been living., finding out where the younger three were born would be my first priority...family, death certs, marriage ???
on line tree has Sarah born Quebec but the source does not say that, say left Canada from Quebec ....she died in Australia in 2004 married name Ludbrook
sorry, no help