Yes get the appointment with the vet and see what your dealing with. Sounds like a blister filled with fluid, but it could come from so many differnt things. Burns, infection, friction/rubbing, just don't know, but it isn't something that should be there, so you need to find out why, and Exactly what it is.

Sorry we can't help , it's hard to know for sure what it is without seeing it , even if we could we would not know the reason behind it. Best left to the experts ( A Vet) Let us know how you make out, hope he heals fast .
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PHEve / Eve