Let's start with details.
Water parameters - I will guess you don't have a testing kit? (Ammonia/nitrite/nitrate).
What is the temperature? It's highly important for a betta to stay at high temperature, 77-80F constant. 75 is already pushing it.
Clamped fins is a symptom of nearly everything. Stress? Parasite? Fungus? Is there ANYTHING physically different about him besides his tails? How often do you change water, and how big is the container he's , or was in? Is that purple plant plastic? The tearing is either tailbiting (but doubtful), or it's been caught and torn on something (I'm lookin' at the plant). Or it could be fin-rot, which is extremely common, especially in dirty waters.
What do you feed him, and how often, and how much? And what water conditioner do you use, or do you leave it out for a day?
Lots more details would be really helpful.