I am sorry for your loss!
Trauma is certainly a possibility, but since there was blood around the mouth and rectum, which is the length of the entire digestive system, it could have been one of the gastrointestinal infections for which sugar gliders are susceptible, such as: pasteurella , staphylococci, streptococci, mycobacteria, clostridium, salmonella , giardia , leptospirosis, and toxoplasmosis. Usually symptoms of inappetance, lethargy, or other unusual behaviors, occur before death, but it is also common for subtle symptoms to be overlooked.
Did he have room to exercise, leap from one spot to another (that's part of their normal behavior)? It sounds like some kind of impact injury that may have caused internal bleeding. It's just a guess without a necropsy and it's been days now so the body would be in no condition for a decent exam. I see this kind of bleeding at work usually caused by an animal being hit by a car or grabbed by a predator, both of which involves crushing injuries. Your glider wouldn't have had that but crashing into a wall? Maybe. . .