Well, if he looks like he's in distress or having trouble swimming, he might have it. It's not fatal, unless you have a very severe case and it doesn't look so bad as far as I can see from the video (a bad case is when the betta is stuck at the surface of swimming on its side or upside down). It usually happens when bettas are overfed, but some are just more prone to it. Remove some of the water so he doesn't have to swim too far to breath and treat with Epsom salt and fasting for a few days.
I plan on doing a water change tomorrow anyway, I just won't fill the tank so much. There's already stress coat in the water to help with his ripped tail, I'm afraid epsom salts would shock his system too much. (Can you tell I'm a first time betta parent?)
I think things are going to get worse before they get better... Started fasting Beau this morning, and his swimming has gone down hill. He's currently resting next to his cave and leaning against it, but when he was up on his leaf-hammock he kept slipping sideways and freaking out.
Ok, I have to ask. Is it possible that he's faking it? When I came home from work this afternoon he was fine, absolutely no problems at all. Now he's having all the same problems again. He's just finishing his second day of fasting, and I'm starting to feel guilty about not feeding him.
Is there anyway this could be temperature related? I've been having trouble regulating the temp in his tank. It's unheated, but with the lid down the temp just keeps rising. I've resorted to propping the lid up an inch or so, and it helps. Thankfully Beau isn't a jumper.
Be very careful leaving the top off although you have never seen him jump that doesnt mean he cant and many Betta are lost cause they have jumped out of their tanks. Better safe than sorry. If you could get a small heater that would better for him to keep the temp of his water more stable. Betta are tropical fish and the temp of their water needs to be at least 76, I keep mine at 79. Good luck keep us updated on how he is doing.