For you guys who are confused :-
मैं उनका बेटा हूँ MEANS I Am His Son(formal = when you respecting your parents, and It's the right way )
I Am His Son(मैं उसका बेटा हूँ), means the same tho. But you should go for it
I think it's correct. It would mean I'm his son or I'm her son.
As AmiSUBHAM implies - it's correct but disrespectful. Like saying तुम rather than आप.