I can't really see the context without hearing the rest of the convo.
Was the kid saying that Stalin was Jewish or could he have been talking about the other kid on the bus being Jewish? Your son could have possibly said
something 'offensive' and replied back saying 'i don't care' in regards to the other kid on the bus...
As I see it "I don't care" is in NO WAY anti-semitic unless it was condoning some kind of put down