Unhappy Chin/Nervous all the time

My rescue boy Nimbus just seems to be on edge all the time. He's had some good days in the time since I've had him (November 6th, and he's 3 years old approx.) but lately he doesn't want to come near me, and doesn't like to be touched (even chin scratches, and he loved those). I keep the room he's in dark, and the door closed, but when someone even walks past the door, he bolts/spooks. (He shares the room with a hamster and two fish, but that's all. I have no other pets.) He has two hideouts currently that he can hide in, but he rarely uses them. He just sort of sits in one spot in his cage all day and sort of appears half-asleep (I've never caught him sleeping for real). I know he's a lot more active at night, chewing on things and bouncing around. He's pooping normally, peeing normally, eating normally... He's eating Oxbow Chinchilla food (two small handfuls a day... I have small hands) and Oxbow Timothy hay (a handful or two... he's kind of picky about which strands he wants, but he's never without food or hay) I've weighed him a small number of times, and he turns up around 600 grams. The highest he's been is 650, the lowest is 550. He's small, but I think that's because he's likely pet-store stock. I don't weigh him very often because he hates being handled, but these are the results I've got from him.

I plan to get him a baby brother eventually (likely not for several months) hoping it'll make him less lonely and more secure...he's never lived with another chin before (he's lived with one in the cage above him) and I've been told an 8 week old baby would be best to pair with him, cause it will seem the least threatening.

Anyway, he just seems really stressed out all the time, and I'm not really sure what to do to put him at ease. Any tips or thoughts are welcome!