I would wait a bit before writing him off as having retardation. I wondered the same thing about Batman but he recently JUMPED and he never did before. He can be fast when he wants to but it's just part of his personality to go slow and act very scared and still.
His history is different than yours, though, because he was about 2 years old when we adopted him and he had 2 cats that stalked him and his previous owner liked to get drunk and chase him around the room.

He was also kept in an abysmally small cage and fed primarily apples. He has rehabilitated immensely.
Idk about your glider but I would just wait and see. It may be that he is just playing submissive in the hopes that other gliders will leave him alone? Not everyone is dominant and they survive by keeping their head down? Keep a journal so you can see his progress.