Possessive forms are always confusing for any average English writer. The book “Elements of Style” starts with this issue. Here’s a small note with appropriate references.
Where to use ‘s?
- Charles’s friend
- Buns’s poems
Where not to use ‘s?
- Jesus’, (if old names end with us, es or is. Seems Charles is not an old name.)
- for conscience’ sake, for righteousness’ sake, Moses’ Laws, Isis’ temple.
- plurals – boys’, girls’, parents’ (note: parent’s and parents’ mean different things… former is singular and latter is plural)
- its, yours, ours, theirs, hers.
- It’s means “it is”.
- its means “belonging to it” (possessive).
- There is no ‘ or s after his.