Hello,
That is an odd formation indeed!
I agree, getting some of the fluid from it aspirated by a vet to culture is the best option to be
sure that it isn't filled with infection.
How long has this been there, did it just appear suddenly?
There isn't any type of blistering disease, no, just the yellow fungus though that is bad enough
of an ailment.
Is his appetite doing alright?
Tracie