phil > Most Russian/Ukrainian immigrants to Israel in the last few decades are not halachically Jewish
Most of Rusim came from 1970s to 1990s. Later arrivals are indeed remnants of those who did not come when everyone else did and are way more likely to be non-Jewish or even, if Jewish, have marginal attachment to Judaism. Same goes with Beta Israel – dedicated Jews came first, converted – after.
If this kid ended up in a yeshiva, I don’t think this is random in Israel where people segregate. Maybe his parents are BTs of some degree and the mother realized at some late point that she is not Jewish and did not find strength to disclose (as that Lebanese chatan)