Assuming that he has diarrhea only one or two times he'll be okay if...
...it isn't "really bad" diarrhea.
...there isn't any blood or worms.
...he's eating mostly normally.
...he's peeing normally.
...he doesn't look sick or behave in a way that makes you think he could be sick.
If all of those things are true you can wait until tomorrow when he goes back to the vet for his second visit. Be sure to tell the vet, though!
But, if any of those bad things happen or if it doesn't go away after one or two times, you should call the vet right away.
...it isn't "really bad" diarrhea.
...there isn't any blood or worms.
...he's eating mostly normally.
...he's peeing normally.
...he doesn't look sick or behave in a way that makes you think he could be sick.
If all of those things are true you can wait until tomorrow when he goes back to the vet for his second visit. Be sure to tell the vet, though!
But, if any of those bad things happen or if it doesn't go away after one or two times, you should call the vet right away.