Is the bubble clear? If it is it may be a fluid-filled cyst that formed there inside his fin, or it could be from bacterial infection. Sometimes cysts aren’t infection at all and just form for no apparent reason just like in land animals.
On the other hand, if it is a round sphere that looks yellowish/whitish it may be an encapsulated parasite. There are some parasites that look like that in guppies but there really isn’t much you can do because the parasite is protected in a dormant stage inside the capsule.
I can't really give you any information without seeing a picture first.
Can you atleast try to get a picture? It doesn't have to be perfect, but just enough to see the 'bubbles'. If a picture is out of the question, a short video would do.