The most useful rule — and the most general and the easiest to remember — is simply that you add ’s whenever you actually say an extra
/əz/ at the end when forming the possessive, compared with how you say the non-possessive version. Let your own ear be your guide. That’s all there is to it. No fancy rules full of exceptions. Just your own ear (as a native speaker, mind you).
So words ending in unstressed
/iːz/ are exempt, like for example this series’ end, that species’ demise, Sophocles’, Socrates’, Achilles’, Diomedes’, Archimedes’, Eratosthenes’, Ulysses’. (But not trapeze’s, because that one is stressed! See how that works?)
But these days, not much else is. I say “in these days” because in previous ages, some people did not add another
/əz/ if it already had one, and so wrote Jesus’ to indicate they did not say an extra
/əz/ there compared with Jesus: both are just
/ˈd͡ʒiːzəs/ However, most people today now say Jesus’s, because it has three syllables:
Same with Moses’s with three syllables instead of the older Moses’ with just two. Note that things like Ross’ and Chaz’ are always wrong, because no one says those with only a single syllable. That is a common error.
So it’s your boss’s house, because it’s got an extra syllable when you say it. Similarly, all the Jameses I have ever personally known have had the extra
/əz/ tacked on when you are talking about something of theirs, which means it is for those speakers James’s house, albeit all the Jameses’ houses, because nouns are only allowed one
/əz/ inflection, not two.
In all cases, the best thing to do is let your own ear be your guide, because writing should represent speech. That means that if you say an extra
/əz/ then you write ’s, but if you don’t say it, then you don’t write it. That’s why you from time to time see forms like for goodness’ sake or for conscience’ sake. Those are possessive, but have no extra syllable.
As for the specific case of Aeneas, in older writing you will find that because his name already ends in
/əz/, people would suppress the extra one when they would form the possessive, like Aeneas’ escape from Troy. Note very carefully that that posits a two-syllable possessive when spoken. If when you yourself say it, however, it turns out that you would yourself use the three-syllable version as most people today now do, then it would have to be Aeneas’s escape from Troy.
But now you have three issississes in a row, which will certainly require careful elocution to pull off — especially if you don’t mean to sound like Gollum with his fisheses.