1.Is this your son?
Yes,it is.
No,it isn't.
2.Is this boy your son?
Yes,he is.
No,he isn't.
3.Is Tom your son?
Yes,he is.
No,he isn't.
关键是看此句主语是谁,然后选用适当的人称代词代替它.
this用it代替,而this boy和Tom都用he代替,回答即可.