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A legal guardian is a person who has the legal authority (and the corresponding duty) to care for the personal and property interests of another person, called a ward. Usually, a person has the status of guardian because the ward is incapable of caring for his or her own interests due to infancy, incapacity, or disability. Most countries and states have laws that provide that the parents of a minor child are the legal guardians of that child, and that the parents can designate who shall become the child's legal in the event of their death.
It is not likely that a court will grant him guardianship because you can act in that capacity. Also, because he is not your husband or their adopted parent, the court would likely have some reservations about awarding him guardianship.