What is wrong with my dog?!?

I have an American Bulldog, and over the last 3-4 months he has gotten these lumps and dots all over his body, they aren’t painful, but some of them burst and they become open wounds, and then become painful. I have taken him to 4 different vets and none of them know exactly what it is, it’s not cancer or anything,... show more I have an American Bulldog, and over the last 3-4 months he has gotten these lumps and dots all over his body, they aren’t painful, but some of them burst and they become open wounds, and then become painful. I have taken him to 4 different vets and none of them know exactly what it is, it’s not cancer or anything, we have had that rules out, but I was wondering if it was something like Pyoderma, or if not, I’d anyone has ever seen or experienced anything like it. I just want to help him, and I don’t know what I can do to accomplish that.
Update: They are less like lumps but more like sores coming up everywhere, and they look like they get better in some places and then they just come back twice as bad in another area. We have been bathing him with medicated dog shampoo, like suggested by a vet, but it did not work and look like it has gotten even worse.
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