Hi! Sometimes Platinum gliders grow lighter over time, and Mosaics have what's called "powdering out." I have seen some go from very dark to nearly solid white. Mosaics can sometimes 'hide', especially to novice owners (I don't know where you got him from originally.) For example, a glider with 1 white whisker or 1 white toe can be a mosaic that no one noticed.. Do you have good pictures of him? There is no known health issue that would cause him to turn white.
The 'average' weight range for sugar gliders is 90-140 grams for an adult male. Some just have bigger bodies than others, so they can be heavier without being fat. If he doesn't have 'boobs' between his front legs, he's likely not too overweight.