Magnuson was adopted as an infant by William Grant and Emma Magnuson in Moorhead, Minnesota; his birth parents were never identified in his lifetime and the underlying birth records are sealed, so no birth name or parentage distinct from his adoptive name is recorded anywhere.
What would resolve this Unsealing the Minnesota birth or adoption records naming his birth parents and any name recorded for him at birth, should such records still exist and become accessible.
OpenGenealogical and adoption-records researchWarren Magnuson (Wikipedia)