The only William John Adair I found was living with Wm &Margaret Adair family. Must be their son.
Census has lots of mistakes in it which could explain William's place of birth. Maybe you could check with canada for a birth record for him. Could be also his mother wated him to be a "citizen" in Canada so she said he was born there when they asked during the census. Anything is possible so you have to look at all angles.