Just tell him the truth, that new babies poo on themselves all the time and it's fine, because like everything else, sick, snot, felt pen ( yes, my ds did draw all over his newborn sister) it washes off, and everything will be just peachy after a nice bath.
I am a youth worker and was rather mortified at work recently when one of the young ladies loudly announced to everyone that women poo whilst pushing their babies out. I had to not only admit it true, but also admit that yes I had, to several dozen curious (and rather horrified) teenagers. It has now become a popular talking subject (among the young people) with lots of what if synarios being suggested.
It really was my preference not to spend my pregnancy discussing with teenagers the possibilities of pooing whilst in labour.