Some chins are more calm then others (and young chinchillas are called kits not puppies
) so long as he has always been more on the calm side and is eating, drinking, pooping, and peeing normally that could just be how he is. The danger of letting them have playtime are because most will go go go and can over do it causing them to exhaust themselves. Kits are also clumsy and can get hurt if they bounce around too much and land wrong. Was he with another chin before you got him, not counting his mother? Was he separated from a litter mate or other kit he had bonded with? If so they can miss their friend/sibling. There are plenty of chins that live all alone for their whole life (20+ years as an only chin) though, so I don't think they really
need to have a cage mate like some other rodents do. Having another chin around though is almost always beneficial. They are herd animals so having another chin around, even if in another cage and they don't get along, can at least give him someone to chat chinchilla to. I also have had chins that enjoyed having playmates but didn't want a cage mate.