how old is he? that is quite a low weight for a chin to start with, he must be tiny! is he in a cage by himself or is he in with others? sometimes it's very difficult to see if they're eating as much as they normally would because there are others in the cage - you may see him eat but if he's eating slower than usual, he will not take in as much food.
given you're taking him to the vet in january, i would be very tempted to try and get his weight up - i have a rescue chin that suffers from health problems and is small to start with so she needs 'fattening up' a little every so often. you could introduce more rolled oats, i would even be tempted to feed him critical care. it's possible he may have a tooth problem - he would need to go under anaesthetic for that and obviously he needs to be a good weight for that. similarly, if it's something else that needs to be treated with medication, a lot of meds act as appetite suppressants so then you get into a vicious circle of him needing to be heavier but not wanting to eat.
keep an eye on his weight, that's important. good job spotting it!
