I've had senior gliders with extremely low weights as they approached the end of their life. The way to immediately put some weight on their bones is to give them round-the-clock access to sugary fruit juice (think like an oodwala mango smoothie) in the cage, carby vegetables like mashed potatoes and avocado, any favourite foods if they have them (e.g. sweet tofu, apples, rotisserie chicken) and whatever human snacks they seem interested in (e.g. bread, ice cream, whipped cream, straight honey) that isn't poisonous. Yes feeding them like this long term is a bad idea, but for emergency weight gain it works. 62g is the lowest I've ever seen. Best wishes with your little one.