FANNING, GREENBERG, late 1800s, RIGA?

Searching for ancestors and relatives of Gustavus Maximillian FANNING born 1 Jan 1880 in Riga, Latvia. Attended Mitau Courland school.On American marriage license, he listed his parents as Henrietta GREENBERG and Gus. FANNING and listed birthplace for his parents and himself as Germany.His father died c. 1890.His mother remarried (date and person unknown) but would have been before c.1895 when Uncle Gus left home and went to sea.Could anyone tell us what any of these names would have been before being anglicizedor help us with leads?We can find no immigration records for him and believe he may have immigrated to U.S. illegally(before 1915).