Whatever it is, move him to another tank immediately. A pleco, even a small one, should never share space in a 2.5 gallon with another fish - there's just not enough water.

Take minaku's advise. The betta alone in a 2.5 is enough. Plecos grow rapidly and need very large tanks. Exception is the smaller ones like "bristlenose plecos" they only get about 4.5". Sorry sounds like dropsy. Another possibility is just bloating from eating to much protien from the left over "meat" foods bettas eat. Usually Malawi Cichlids get this as thay are primarily vegetarians and get "bloated" from eating too much meat proteins if kept with carnivores. It is possible for other species to get this and it is deadly if not treated. Isolate the fish anyway for treatment. One distinct symtom of dropsy is protrudeing scales along with the bloat. The link below is kind of a PITA to navigate but has listed treatments.

Here is a link that might be useful: fishy farmacy