J.S.
Wow. I've lived in several houses with these type of vents, but they never had one in the bathroom. Any powder would most likely end in disaster, with it being blown all over your house later this year.
Maybe you already did this, but if not, take off the vent cover. Clean or soak it, and clean in the vent as far as possible with vinegar on a rag. If your son is healthy & well-hydrated (and only did this once?), the smell would probably dissipate before it gets too cold. But if not, those pet sprays work pretty well.