However if the OP's son got on at a station which had no ticketing facilities, then it is very relevant with regards to a PFN, and actually makes the fact their son did not intend to travel in 1st Class irrelevant. I can't recall the OP stating at what station his/her son boarded the train.
As I mentioned, its unlikely that they did board at a station with no ticketing facilities, but until we know for certain, we can't dismiss it.
Had the passenger boarded at a station with no ticketing facilities, I think it may have been mentioned already. I am ready to dismiss the possibility unless the OP clearly says otherwise.