In the sentence
Wo soll er denn hin?
what is the main verb? "Soll" is an auxiliary verb and "hin" is the verb meaning "go" in English. (My book explains it that "hin" is an adverb here which seems wrong.)
Of course in "Wo gehst du hin?", "hin" is an adverb.
Isn't my understanding correct? I need to confirm.