According to the Bible (Genesis 15:2-3) he was a servant of Abram/Abraham (traditionally ca. 1838-1613 BCE) and probably was adopted by him in accordance with a fairly common middle eastern practice. A childless or son-less couple would adopt a male servant or slave in order to have someone to care for them in their old age and to perform the necessary funeral and mourning rituals. In return the adoptee would inherit their estate. If the couple subsequently had a son (as did Abraham and Sarah) the adoptee would become a secondary heir. (Compare the entry for Roman Empire.)

References

The Complete Who's Who in the Bible, edited by Paul Gardner. (London: Marshall Pickering, 1995)
Encyclopaedia Judaica, edited by Cecil Roth. (Jerusalem: Keter Publishing House, 1971)

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Figures Whose Adoption or Fostering Is Fictitious, Disputed or Unconfirmed
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To Provide Heirs, As Protégés, etc.
Wealthy, Famous, Noble or Divine Adoptive or Foster Families
Customary or Traditional Adoption, Informal and Extra-Legal Care
Institutional Care
Parent(s) Died, Disappeared or Became Incapacitated