Don't listen to those who feed twice a week and once a week. THEY LIE! Why? Okay, let's take my little Admiral, the rescue for instance. HE WAS MALNOURISHED. He didn't make it. He lacked the nutrients a healthy betta needed to fight for his wee life. I feed mine every day, depending on my schedule once or twice (2-3 pellets per feeding), and a day of fasting.
YES bettas can survive
without food for weeks on end... But it's some ignorant people spreading terribl rumors that cause the fish to survive not thrive.
I don't think your fishy is clamping his fins. He's a veiltail, right? Most veils (I will add two pictures) will pause with their fins dropped and may SEEM clamped, but they aren't =D Then when they swim they may hold their fins very grand. Some veils don't ever hold their fins grand.
First picture: Juniper chilling out. His caudal had rot before, afterwards it wasn't bad. He never really held his fins well unless he was flaring!
Second picture: Maine. He held his fins very well while scouting his territory, flaring, and flirting
LAST pictures, is clamping.