I would say "it" if you cannot avoid it... otherwise, I would try and say "the cat/kitten" instead of him, her OR it. "It" is the correct use of English though - saying him/his is misleading and just as offensive to the female cats as it would be to human women to be referred to as "him/his". hehe

Now, if you're talking about different cats and they have names and genders, use their name and then you can say things like "his toy" or "her food" - but only after you've stated the name and gender.

English. It's so darn confusing. Ain't it great?