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
And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it.
1991, Robert Harling and Andrew Bergman, Soapdish, Paramount Pictures
Celeste Talbert: She is my rock, my right hand.
If A depends on B, then B provides for A and B is a provider.
to make arrangements for supplying means of support, money, etc. (usually followed by for): He provided for his children in his will.
to supply means of support (often followed by for): http://www.dictionary.com/browse/provide
- supporter If A depends on B, then B supports A and B is a supporter.
- supplier If A depends on B, then B supplies A and B is a supplier.
- caregiver If A depends on B, then B cares for A and B is a caregiver.
Simple language ordinarily trumps oral frippery (cf. Hemingway's "poor Faulkner" comeback), as exemplified by the first part of this sentence, but... There, are, times. So, thusly..
A generous patron, esp. of art or literature.
A person who supports or helps a person, institution, etc., esp by giving money; patron.
Not necessarily the most general alternative, but sponsor is sometimes used.
The Medicis were Galileo's sponsors.
If the support is monetary and public, patron can also be used.
The Medicis were Galileo's patrons. The NEA was Billingham's patron.
If the support is 'active', then advocate can be appropriate.
The indigent depended upon his advocate to navigate the bureaucracy.
For a generic direct straight counterpart to "dependent," there was nothing. But in computer science and technical land, your "dependent" has a "dependency" on you, the "dependee."
So, for the most generic (abstract) direct counterpart to "dependent," it could be "dependee" if you were brave enough to help propagate it to more general use over time.
A not-great source is here, https://en.wiktionary.org/wiki/dependee, but I feel like it does not do the (newish) word justice.