Increase the water supply to the tree once the fruitlets have formed by watering it yourself, making sure it gets a couple of gallons every 5 days, or by leaving the hose running on a trickle at the base for an hour every 5 days.

I agree with Bamboo, they do need watering through dry spells, but these cherries need lime, They won't grow proper fruit if on acidic soil, nor will they respond very well to being watered, if in a pot, with rain water, which tends towards to be more acidic. The fruits form, but the stones do not grow, which keeps them small. This is where the lime comes in, it is what helps them form the stones inside
I have the same problem, I sprinkle lime round one in the garden, The other 2 I have planted next to a wall, and I give both of them lime in water during spring and early summer. A tablespoonful in a watering can around the roots.