I hate it when replies get snuffed out, Glo. When I write a long one these days I copy it before sending it, just in case.
Hi Kei, good to see you again. You might want to try a different gravel, or keep your gravel cleaner, or remove it entirely. When tails drag on it (some bettas like doing that), they get ripped, and in less than pristine water conditions fungus sets in immediatley.
About cutting the tail - yes, some people do that. I don't recommend it really - too easy to make a mistake. Salt is really good about killing fungus, and bettafix promotes good healing. try moving your little one to a hospital tank, get warmer temps and clean water. Add salt and bettafix. When he's well, consider keeping him gravel free, since he is so prone to finrot. let us know how it goes!