Did Julius have another son?

We've always known that Julius Rabinowitz and his wife Annie had two children—Abner, born in 1907, and much younger Judith, born in 1919. But look at these census records for the Julius Rabinowitz family in 1920 and 1930. On both forms, look at the top rows, which show only some family members (the others are on bottom of the previous pages, which I also have).

Anyway, you will see Abner and Judith on both forms, but also a Seymour, apparently born in 1903. That's weird (1) because we haven't heard of him and (2) he would have a living uncle named Seymour. But see for yourself (click to enlarge).

What do we make of this?

Small update: I can't locate a census record for Julius and family in 1910. He is not listed with the Joseph Rabinowitz household in that year, which makes sense because he is married with children (one or two).