I would create a new class. A character whose only special ability is to possess animals while asleep isn't really supported.
Perhaps you could get away with a druid that only has wild shape powers, which represent the abilities the character has when inside an animal. The character wouldn't actually change shape, though. Maybe replace the Wild Shape power with something like this:
Skinwalking * At-Will
Standard Action
You can make a Wisdom vs Will attack vs a beast within LOS. No roll is required if the beast is bonded to you. If the attack succeeds, your humanoid body becomes unconscious and your mind now controls the beast. While controlling the beast, you can use druid evocations with the Wild Shape keyword, but cannot use other class powers.
For Jon Snow I would make a beastmaster ranger.
Note: I have not read the latest book yet, so I don't know anything that's in there.
