Son had loose fecal accident in small unchlorinated baby pool. Did not know until after he swam in it for an hour. Should I call doc? Need meds?

Observe for now. Children or older people can catch certain waterborne illnesses (e.g. giardia, cryptosporidium E. coli etc) from contaminated water, that is Unchlorinated. If he develops any symptoms in a few days like diarrhea, abdominal cramps fever or vomiting, get him checked to rule out some of those conditions.
Observe. Does not need any treatment now, but keep an eye on any new symptoms like fever, diarrhea, vomiting. If hd ford develop these, see your pediatrician. All the very best.
Stool in pool. Give him a good bath, keep him on bread, rice, apple, bananas, keep him hydrated with pedialyte or other clear fluids depending on his age, watch him for signs of illness, like fever, refusing to eat, vomiting, persistent diarrhea then you should take him to his pediatrician to R / O catching any infections from that contaminated water. Good luck.