keep a close eye on his tail, watch for the skin color to change from light to dark to black. Also watch for broken skin on the tail, if he chews on it get him into an e collar.
The head is most def over groomed, however gliders don't typically over groom their tails just for the sake of over grooming.
Usually if the tail is missing fur it's because the glider suffered a tail injury and it's causing pain.