's

This is an extremely common contraction* which is even used in speech when it's not contracted in writing. In other words, when reading aloud, most people will read "He is all I have" as He's all I have because it sounds more natural and has a better rhythm.

It stands for:

⦿ is when followed by a predicate:

he's a doctor

she's here

who's in the kitchen

there's sugar

⦿ has when followed by a past participle:

he's been there

she's spoken to me about it

anyone, who's given money

⦿ does only after interrogative words in questions** like How's it feel? meaning How does it feel? and Why's he do this? meaning Why does he do this?

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*The forms -'s and -s' are also added to nouns to show POSSESSION, but we won't treat them here because these are suffixes, not contractions.**Note that this is normally a spoken contraction and not a standard written contraction. Even though you'll see it in chats and comments, it's a reflection of how people speak and not an accepted contraction to be used in any kind of formal or even informal writing unless it's in a dialog of some sort.

it's vs its

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Be careful to distinguish it's meaning it is or it has from the possessive form its = belonging to it:

⦿ It's raining cats and dogs.

⦿ It's taken three weeks to paint this.

⦿ A dog always knows its owner's voice.

who's vs whose