"Si/sin/sitt" points back to the subject of the sentence, in this case making it clear that the subject ("han") loves his own wife. Using "hans" would imply that he loves somebody else's wife.
The English "his" is ambiguous, so when translating from English to Norwegian we often have to allow for both options.
According to what we learned, it is logical to say "Han elsker kona si", because "kona" is feminine (and has the definite form). Some sentences ago, however, I think I read "Han elsker kona sin" which - should it be correct - I do not understand. Should it be "konen sin"?? Or is "sin" simply wrong as it is the masculine form? I should appreciate a useful hint or explanation.