Hi megladonsharky,
I agree with the others who said that 2.5 gallons is too small for a betta plus some other fish. Is the betta in the half gallon bowl now? I don't think he'll be happy in there for long. Like Menagerie said, put the male betta in a 5 gallon tank, with a heater and a filter. My bettas are in 5 gallon tanks, and I have no health issues at all with them. I have a Visi-Therm Deluxe 25-watt heater, and a sponge filter (Azoo brand) run by an air pump through a gang valve to lessen the water flow and current. Bettas don't like current. I can adjust the gang valve so excess air escapes through some of the unused ports in the gang valve, and air coming through the sponge filter doesn't create a current.
I have found that a big factor in finrot for bettas is the water temperature. Without a heater, the temperature fluctuates, and over time, just a fluctuation of 2-4 degrees throughout the day and night is enough to stress, and cause illness, in the betta. 80-82 degrees is the optimal temperature for a betta.
One reason your betta didn't seem to like the 5 gallon before may have been the decorations in the tank, or possibly the current was too much for him. I have a mix of real and silk plants in my tank. Bettas like to explore, and also like to find a special "spot" to rest. If you fill the tank with 5-7 plants, I bet the betta will be very happy. Remember to use silk and not plastic plants for bettas. Their long, delicate fins can tear on plastic decorations.
Some medicines that I've found useful for bettas are BettaMax and Jungle Fungus Eliminator. If the betta has finrot, but is still active and eating, try BettaMax. If he is lethargic, or the finrot is especially bad, try the Jungle product. You can make a tonic with the BettaMax as it suggests on the box, and do daily water changes with the BettaMax for 5-7 days. The Jungle Fungus Eliminator suggests a water change 4 days after the initial dose, but when I used it I did a water change every other day, and treated the replacement water with the amount of medicine that I was taking out.
Unfortunately, my bettas don't make bubble nests in the 5 gallon tanks, with the air pump, even though the top of the water doesn't move too much. I can see that they are happy, and healthy, so I wouldn't consider putting them in a smaller container just so they would make a bubble nest.