According to the peewee expert, "peewees do that, and there's not much you can do to help." They can be fine in the morning and in serious trouble in the afternoon. Sometimes they get a kidney infection that affects the sciatic nerve.
All I can do is keep the fluids going. I've made the food pretty mushy and dip it in water just before I feed the bird. If I use small balls of food I can get quite a lot of water in during a feed. His cage has been wrapped in towels to keep the breezes out of one end, so he has a warm and dark area to be in, or a warm and sunny area to sunbake.
But I'm not very confident he'll survive.