6.24 The general rule for the possessive of nouns covers most proper nouns, including most names ending in sibilants."
Examples they give include Kansas's, Ross's land, and Jones's reputation. Exceptions include Jesus' and Moses'.
Which names does this apply to? Is the Aeneas' form correct, or is it Aeneas's instead?
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).
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: /ˈd͡ʒiːzəsəz/.
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 three-syllable possessive when spoken. If when you yourself say it, however, it turns out that you would yourself use the four-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.
There are four exceptions to this rule: (1) The possessives of personal pronouns do not take apostrophes (ours, yours, its, theirs). ... (2) Biblical and Classical names that end with a /zəs/ or /eez/ sound take only the apostrophe: Aristophanes' plays[;] Jesus' suffering[;] Moses' discovery[;] Xerxes' writings[.] No extra syllable is added in sounding the possessive form. (3) If a corporate or similar name is formed from a plural word, it takes only the apostrophe. Thus General Motors makes General Motors', not General Motors's ... (4) According to traditional rules, a sibilant possessive before sake takes merely an apostrophe, without an additional -s—hence for appearance' sake, for goodness' sake, and for conscience' sake.
7.21 An alternative practice for words ending in "s." Some writers and publishers prefer the system, formerly more common, of simply omitting the possessive s on all words ending in s—hence "Dylan Thomas' poetry," "Etta James' singing," and "that business' main concern." Though easy to apply and economical, such usage disregards pronunciation and is therefore not recommended by Chicago.
Of course, Chicago's new guideline 7.18 likewise disregards pronunciation—and yet it is not therefore not recommended by Chicago.
The problem with this treatment is that all of the "difficult pronunciation" examples involve vowel-s-vowel-s names (Moses, Isis, Jesus) and all of the "ancient classical names" examples involve mythological figures (Mars, Achilles, Hercules), so we don't have explicit guidance on whether Euripides (for example) would qualify as "difficult to pronounce" in possessive form (probably not) or as an "ancient classical name" (maybe).
SINGULAR PROPER NAMES ENDING IN S: Use only an apostrophe: Achilles' heel, Agnes' book, Ceres' rites, Descartes' theories, Dickens' novels, Euripides' dramas, Hercules' labors, Jesus' life, Jules' seat, Kansas's schools, Moses' law, Socrates' life, Tennessee Williams' plays, Xerxes' armies.
Use 's after non-classical or non-classicizing personal names ending in an s or z sound: Charles's[;] Marx's[;] Dicken's[;] Leibnitz's[;] Onassis's[;] Zachariasis[;] Collins's[;] Tobias's[.] While convention allows latitude in possessives (e.g. the additional s is used more in speech than in writing), the possessive misconstruction Charles Dicken's is always incorrect.
An apostrophe alone is also permissible in after longer non-classical or non-classicizing names that are not accented on the last or penultimate syllable: Nicholas'(s)[;] Barnabas'(s)[;] Augustus'(s)[.] Jesus's is acceptable in non-liturgical use. Jesus' is an accepted archaism—Good friend for Jesus' sake forbear—and Jesu's is also possible in older contexts.
Use 's after French names ending in silent s or x, when used possessively in English: Dumas's[;] Descartes's[;] Hanotaux's[;] Crémieux's[;] Lorilleux's[.] However, since appending the plural s would be grotesque (Lorilleuxs) or misleading (Dumass), the singular possessive is treated like the plural, for example both Lorilleux's (not Lorilleuxs') cat, the two Dumas's (not Dumass') novels.
This list shows considerable support for Aeneas', but it is worth noticing that both Garner and the current Chicago—which carry considerable authority in publishing houses in the United States—favor Aeneas's. If you are supposed to follow one of these style guides, your course is clear; if not, you have some research to do (if some other authority controls your style choices) or a decision to make (if you're on your own).
The simplest rule is to always add 's to a singular noun. It is traditional to omit it on a few words, but it's never wrong to add it. Since you don't have to pronounce the 's, how the word sounds has nothing to do with it.
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The Jones's, Joneses, or Jones'?