D.B.
The nurse gave you standard advice. A plum pit is small (not at all like a peach pit), and it's smooth. You probably won't even notice it when it's eliminated - it will likely be right in the middle of his stool. Kids do this all the time - I know it feels different when it's your kid and your plum pit, but I think there's a very low likelihood that there will be any problem whatsoever.