My yorkie gets them on his stomach and it is from allergies. It's a condition called Pyoderma. You can google Pyoderma and read about it. If it gets bad, your vet can prescribe antibiotics and special shampoo for him. Beau even gets them on his back towards his tail sometimes. I do understand how you feel because the first time he got them, I thought they were ant bites or something.
I definitely do not think that your yorkie has a pimple.