I'm no expert, but could he be sulking? From my experience in my short time owing a betta, they definitely have feisty little personalities and I wouldn't be surprised if he's just mad at having to share his space. Either that, or he might be intimidated. I know bettas are supposed to be aggressive but my neon tetras had put my guy in his place because they would always bite his tail and he would jump away and cower. lol You know, the whole stregth in numbers thing. There are 5 shrimps and one Tokyo so he's smart to the possibility of them ganging up on him.
Hopefully after a few days, they will get used to being around each other and no one will hurt the other.