What does 'come' mean in this context? [duplicate]

"Come" means arrived. In today's English one would say "Has he arrived?", but this forecloses the wordplay that Shakespeare is making (between understanding "coming" as arrival and journey, the process that leads to arrival).

"Is" is there instead of "has" because of antiquated rules for forming the past participles of certain verbs that denote changes of state (one sees this still in French and German, at least).

The line can be reworded either as "Has he come?" or "Is he here?". Your understanding is correct, it is just "old timey". As for any rule about it, I am sorry I have nothing there.

Evidence was requested. Here's what I found {ODO}:

definition of to be:

archaic Used with the past participle of intransitive verbs to form perfect tenses: I am returned; all humanity is fallen .