I'm concerned that there may be a misunderstanding with regard to terminology.
If your son needs to assign a "part of speech" to each word, his choices, according to many classifying schemes, are:
noun, verb, adjective, adverb, preposition, conjunction, interjection, ...
The following are not "parts of speech": subject, object, indirect object, object of a preposition, clause, ...
In terms of parts of speech, "Topeka" is a noun, and "Kansas" is a noun, because both are names of places.
I hope this helps. Please respond if you feel this reply is inadequate.