Poor guy. He must be very bewildered as to why his body keeps flipping him over. Yep. Wry neck.
It can come on suddenly from a blip in his nutrient balances. One day he's got enough vitamin E in his system and the next day, wham! A shortage is flipping him on his head!
If you have some vitamin E oil 400iu capsules on hand, pop one in his beak. Do it each day until his balance, both literally and nutritionally are restored. If you can get some selenium, add a sliver to the E and it will absorb better.