For now, no ped -- and I'm someone who truly believes in going to the doctor when there's any doubt whatsoever. The one troubling thing is the single bout of diarrhea. Was it truly runny, I-can't-control-it diarrhea or maybe just a very loose poop? If he really had diarrhea he likely would have had more than one bout already.
However, stomach viruses can be very strange and I know my own child has had viruses where she would vomit once, not vomit for 20 hours then vomit again - and the doc confirmed that some viruses do act that way and don't cause constant vomiting or ongoing diarrhea. So if he really did have diarrhea, I'd keep him home from any activities around other kids for a least 24 more hours; he could have another bout if it's the kind of virus that behaves that way.
The nororvirus is racing around like wildfire in our region right now, and though your son's symptoms don't sound like that at all, his resistance may be low due to the cold, so I would keep him out of play places for several days. My friend's son had a cold and was low and draggy and ended up with a separate bout of norovirus that was much worse on him, the pediatrician said, becaus the cold had made him weaker.
If your son has a second round of diarrhea or starts the vomiting/diarrhea routine I would phone the doctor first and see if they want him to come in.