I also have an older man in my family tree, who came to the US in 1910, and then seemed to disappear. He said he was coming to see his daughter in Trenton NJ on the ship manifest, but he never showed up on a census list, or in a death index, or in any cemetery. It turns out that he went back to the old country, to Lida (now in Belarus), where I found him on a tax list in 1912. This man also left a wife behind in Lida, though the grand majority of the couple's children were in the US by then.
You don't say how old your Nissen was, but I take it he was 50+ years old. Perhaps he and/or his wife felt it was too much effort to start a new life over again in Montreal, and so it is a real possibility that he did go back. There aren't any outbound ship passenger lists from the US to Europe that you can check, nor are there any inbound ship lists for arrivals in Bremen or Hamburg (so unfortunate). You could try and check the New York City death records and indexes (via SteveMorse.org), to see if your Nissin shows up there.
Hope this helps.
Lak/Lok/Liak/Lock and Kalon/Kolon in Zagare/Joniskis/Gruzdziai, Lithuania
Lak/Lok/Liak/Lock in Plunge/Telsiai in Lithuania
Rabinowitz in Papile, Lithuania and Riga, Latvia
Trisinsky/Trushinsky/Sturisky and Leybman in Dotnuva, Lithuania
Olitsky in Alytus, Suwalki, Poland/Lithuania
Gutman/Goodman in Czestochowa, Poland
Lavine/Lev/Lew in Trenton, New Jersey and Lida/Vilna gub., Belarus