What is this??

Anybody know what this could be? Is it a scab? An infection? It's on my dog's stomach, but I don't think he did this himself, because it's a spot he can't reach to lick.

It's hardened, and I thought it may have been a popped pimple so I tried to wipe off the (pus?) but it doesn't wash off. Btw, he's gotten a pimple or two in his lifetime, so I assumed it may have been one.

Also, it doesn't seem to be bothering him in any way- so I took this discussion here for opinions, before bringing him to the vet.