Some possible explanations
A) for some reasons he lied in the census. Did he declare the same in other censuses?
B) the census taker made a mistake (not unusual)
C) he was indeed a german speaker. Perhaps he had partly german origins.
D) he was born in a place close to the Russian German border,
You didn't give his name, have you tried to find any immigration or naturalization documents ? They will certainly give you some definite answers.
Happy new year !