Hi!
Tiger barbs like to be in schools. When I first got them, I had only 3, and the biggest picked on the other two, which would hide from him as best they could. (Probably why yours keeps hiding). I added 4 more to the tank, and they are all happily co-existing now. They still chase each other, but it is much more playful, and no one fish seems picked on. They are a lot of fun to watch. I've noticed that when mine sleep, they have their heads down a little. My water levels are fine, and the fish seem happy and healthy, so I think it is just how they rest.
I would make sure that there are plenty of plants and other hiding spots where they can hide without getting hurt, and if you have room, I'd get 4 or 5 more Tigers so that your one fish isn't picked on. If he doesn't get a break, he could get stressed and sick. Also, they are less likely to pick on other fish if they have a group to hang out with.
This is just from my experience, so maybe some other Tiger barb people will have some more advice... Good luck!