"Uneasy" is the right word for it, it was driving me nuts because nothing really seemed "wrong," but you can tell when they're feeling off. He's been here less than 3 months so I wasn't even sure his personality isn't just prone to being sulky, especially since he's probably going through puberty currently. And I know what you mean, something about the chunky soft body on a bp makes them look extra relaxed and cozy when they sleep!
I have a few things I got for him to try out, but since he was under the weather I was backing off on enrichment type things for awhile to try to calm him down. He likes curling up in the dog's blankets so I got him some fuzzy hammocks and things intended for rats and ferrets - I'm looking forward to his sweeter personality coming back and hopefully he'll want to interact more. I've also heard of people setting up a playpen or jungle gym outside of the enclosure for them to play on - he's in a sterilite so there's not a ton of overhead space to add things, though it is easy to drill new holes to attach and hang new stuff.