I tried to do some quick research but there is not a lot to go on.
Do you know anything about his life in the US before he married your mother? Since he would have been about 55 at the time, was he married before? Did he have a family before. Anything might lead to clues to his origins. It would help to know more precisely when he came here and earlier census records would give hints.
There are also a lot of Lutheran Church records available now if he stayed Lutheran and it would help find him in those records if he had an earlier family.
Do you know the name of the disinherited sister? Do you know the names of any other siblings?
Do you know the name of the uncle and aunt he came here with?