Sia means "his own", and not just "his" in general (e.g. refering to another man's son). Since there is no context or background for other males in this sentence, "lia" would most likely have been perfect to - since it can only be his own in this case.
HAHAHHA "furiously proud"!! love it! And yes, i had a struggle with that one too... but then it looks like the french "fier" for proud, so at least, if you ever look into learning French... could come in handy lol