Betta Fish: Lump on the gill (I don’t know what to do!)

First time betta owner here! I have had this boy for about 10 months now. Our family is familiar with other fish but this is our first time adopting a betta. This lump on his right gill started appearing two weeks ago. I clean the tank and dechloronise, do everything I can thinking it is infection. But it doesn't disappear or reduce in size. He is fed once a day for every day and sometimes twice every 3 days. His tank is 5.5 gallons with big java fern in the middle of tank. The aquarium salt is added every time water is changed which is every 10-15 days. It has filtre but not heater because I live in tropical country where these guys are native. The weather is already hot enough that I sometimes even have to lower the water temperature. He has been swimming less quickly lately and sometimes just lying at the bottom(last pic) or just staying like a stone near the surface(3rd pic). His tank in the last picture looks smaller because he is in temporary tank since his usual one has to be rescued from small worms (I don't know what those are). I am thinking maybe because of those lil worms. He does eat every time I feed him though. I don't want to lose him just in case this is a big problem. Please help me out.