Are you his dependency, dependendee or something else? What is this type of opposite word called? I believe it is not called antonym, as that would be independent.
I'd say you are his/her support or supporter.
See here and scroll down to the "noun" section.
If someone depends on you and you have a legal obligation to protect, support or care for them and their interests, then you are their legal guardian. The person who depends on you is your ward or dependant.
An informal word for the person who you depend on is your rock.
1611, King James Bible, Matthew 16:18
1991, Robert Harling and Andrew Bergman, Soapdish, Paramount Pictures
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