STOP...obviously the Iaito is not the std. Zn/Al alloy but carbon steel. or am I wrong?
If his Iaito is of value or suspected value (say a Gunto of unknown worth) rust removal is for professionals. One can try cutting open the used Uchiko ball, use a felt cloth and some Uchiko powder (i.e. finest grit sandpaper ever homemade) and rub the rust (having great care to not get cut), clean, check, repeat.
BUT more than likely you didnt buy son Nihonto for Iaido class, any hoo...its still good practice for one day he may own Nihonto.
where is rust? how is sword stored? is Uchiko, rice paper used to clean old oil 1st? Heres a rust maker-what wood is saya made of Honoki wood, if not saya may be rust maker.
hope I helped -Karl. Peace.
do not try to spork the post, for that is impossible, only realize there is no post to spork