I am assuming your cats are a breed of Sphynx.
This breed is very sensitive to the sun, they can get sunburn from sitting in a window trying to get warm from the sunshine. Sunburn can cause blisters that puss.
I just say this because many people don't know that these cats need their environment to be warmer. They do have a 'peach fuzz' coat but it does not keep them warm.
They also need to be bathed at least 2x a month because of the oils that build up on their skin that are not absorbed into hair that a haired cat would. This build up of oils will cause a breakdown of certian protiens that protect the skin and could cause blistering.
A visit to the Vet is a good idea. It has probably become a bacterial infection by now. Would like to know what the Vet says.
No hair? Is it the breed or did they just lose all of their hair?
What you describe isn't good at all and it is potentially painful for them. Please have them checked with a vet or ask on the expert forum for an explanation. I'm sure they will want you to take your cats to the vet as well, but would probably give you more options. I would not refuse blood work. I think it is imperative that you take action before this condition worsens.
Good Luck!
I totally agree with AnnieBrooke, It is obvious the cat as an infection and the abssesses will just go worse and more of them. Abssesses grow inwards in feline, so the infection can be very deep, even though you can see this hole, it could be coming from any where, plz get puzzy to the vet.
Good Luck, Hope All Goes Well.
Abscesses, or one abscess that has given him an overall infection under the skin, would be my guess. Get him to the vet, it's possible to wait too long when a cat has an infection.