William Charles Elliot, born in England in 1884, came to the United States as an alien in 1916 and settled in Boston with his wife Anna, who was born in Belgium in 1879 (her parents were from Hungary) and their daughter, who was born in Canada in 1914. He never got citizenship. Does anyone know what uniform he is wearing in the picture? I can't figure it out.
The cap looks like a form of a Balmoral Bonnet, which you can see quite similar ones on some of the photos of members of the 185th Battalion, Cape Breton Highlander unit here (out of Nova Scotia). If we had better detail on the cap, the cap badge might help us verify unit designation. If you know where in Canada the daughter was born, it might help narrow down or confirm the unit.
So far I have not found anything specific to this swagger stick. I would expect to see a protective leather tip on one actually meant to be used as a riding crop, and if this individual is a member of the 185th, they were an infantry battalion. (As mentioned in comments its could just be a prop since this is a studio picture.)
A second Highlander unit active at that time frame, was also an infantry battalion, the 15th Battalion (48th Highlanders of Canada), CEF. (I lean towards the Nova Scotia unit since the OP mentioned the ancestor settled in Boston.)