He's lonely. They're much more comfortable in groups. I have 5 myself and I still think it's too few. They are still a bit skittish. I've heard they get more calm with larger shoals. Like at least 7. It took a couple weeks before they would stop scattering when I entered the room or walked by the tank.
Also, what are you feeding it? They don't often take to prepared foods like flakes, pellets. I feed mine frozen food (brine shrimp, mysis, bloodworms, glass worms, daphnia) and they love it. They actually gorge themselves until their stomachs are little round gray marbles. A couple of them will take flakes, but I don't use them as a staple for any of my fish anyway.
What are your water parameters? Ammonia, nitrite, nitrate. They are pretty sensitive and delicate to water quality. That's why I change my water religiously, replacing at least 50% weekly.
One last thing, they don't like bright light. They need broad-leaved plants or caves to hide in. Mine usually stay in their large clay flower pot during the day and only come out to explore after lights out. I replaced my large root ornament with the larger pot to accomodate all of them comfortably. They love it!
