There's three parts to your question.
- "come six o'clock" roughly means "when six o'clock comes". This structure can be used to talk about various things that arrive in an abstract way:
"Come fall, we'll be able to harvest these pumpkins."
"My son's coming 18 and off to university."
"Come graduation, I'm moving away."
It cannot be used for physical things, like:
*"Come the asteroid, ..."
*"Come the taxi, ..."
- "rolls on in" is the phrasal verb "rolls in", with "on" inserted, which sounds more casual, friendly or "folksy". With many phrasal verbs, it's possible to add "on" between the verb and the particle:
"Come on in whenever you're ready."
"I'm gonna head on out after this drink."
"The red team is really moving on up the leader board."
It's not possible to insert "on" in phrasal verbs with "on" or "off" as the particle:
"*Put this jacket on on"
"*I told him on off"
Speakers of some US dialects nearly always insert "on", regardless of context.
- Here, "Been to the zoo" means, "I've been to the zoo". This is a quote of what "he" will say when he arrives at six o'clock. For stylistic reasons, the writer chose not the put quotation marks around this sentence.