[DewarHolmes]

I was watching Ponyo in Japanese yesterday and I noticed that Sosuke + his mom call his dad by name （こいち）, instead of saying おとうさん or ちち.
By the same token, Sosuke calls his mom りさ instead of おかあさん or はは. I'm not positive, but he might have referred to himself by name at one point as well.

Can anyone offer an explanation for this? Thanks in advance!

PS.Also-perhaps because of their closeness-he never says りささんorこいちさん.