I don't think it's anything to be worried about to be honest. Cyanide, while deadly, isn't very deadly in small proportions and a cherry isn't going to be enough to kill him. If I recall correctly cyanide works by binding to the hemoglobin and blocking oxygen from being carried to the cells. This doesn't result in death until 60%+ of the hemoglobin in the body is bound. If it's a really small dog then I suppose it's plausible. But really I think your dog will be fine.
If you're concerned or see any strange symptoms then yah, definitely call the vet, but really I think he'll be fine.
Cyanide is formed in rapidly growing forages. We probably consume a bit of it ourselves. But it does dissipate on its own (oxidation) over time out of plant matter.
Also this is assuming that the body can even get into the pit itself. Non-ruminants (humans, dogs, cats, etc) weren't really meant to break down fibers and cellulose, which would be what the outside of the seed would consist of. Ruminants (cows) do a good job of that though. That's why they can eat just grass and survive (well that and the extensive amounts of bacteria in their digestive tract, but that's another topic).