How many times do I have to point out that it's obviously an opinion (Gosh what a highlight, I thought I'd put it in bold so you could see it more clearly if it isn't obvious enough for you ) I have never ( I can use the Italic button as well, amazing ) said bettas MUST have plants or they will die on the spot, I have stated that most bettas do like a densely planted tank, which is true at least in my own experience , and also in the experiences of others here .
If like most shop sold bettas, that betta has come from a smaller container or tank and has been plunked in what is to him a vast open space, it's hardly surprising he's not happy. It's like dropping an agoraphobic in the middle of an open field with only a couple of trees to shelter under.
Having plenty of cover, as in more than a few plants dotted about, will help him hide and settle and have his own little jungle to feel safe in. Even if the entire tank isn't planted, planting up a quater or half will help him calm down and stop sulking and drifting at the surface.