If it is a stroke, don't give up on him! Birds can recover from brain damage better than us mammals.
Years ago, and on a different board, there was someone with a cherry head conure who became partially paralyzed after a stroke just like your cockatiel. His owner had to make special a special set up with some low ramps so he could get around a little and did physical therapy for him (moving his legs for him) and he ended up recovering very well eventually... not 100% but a lot better.