Q. My young son is left-handed, with which hand should I teach him to lift the becher to make kiddush?

A. Shulchan Aruch (O.H. 183: 5) rules that a left-handed individual holds the koss of birchas hamozon in his dominant left hand. However, Magen Avrohom (ibid. 9) questions that the Mechaver contradicts himself since in regards to a lulav (O.H. 651) he behaves as the rest of the people and holds the esrog in his dominant left.
A number of explanations are given, (Ohr Letzion (2: 20) maintains that there is a difference between blessings on mitzvos, on which it is important to honor the mitzva, therefore the left-handed relinquishes his nature and behaves as everyone else. When reciting birchas hanehenin, that by definition are a recognition and a declaration of thanks for a pleasure one enjoys, he uses the left hand since he is most comfortable with it.
Imrei Shem (10: 5; p. 126) asserts that for all purposes, the ruling on lulav is main. The reason why birchaz hamozon is different is simply to avoid spilling the wine, especially when reciting a lengthy brocho, as bentching is.
Horav Shlomo Miller’s Shlit’a opinion is the you should teach the lefty youngster to use for kiddush and bentching his dominant left hand as the Remo and Mishna Berura (ibid.) rule.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a