Your son's primary doctor could run a few basic tests like erythrocyte sedimentation rate (though it's a nonspecific test) to see if there is an infection. Pred can mask the signs of infection.

How long has he been on the Pred? What else is he on? Did he recently have anything like IVIG or plasmapheresis? Have they checked his D-Dimer (to look for signs of a clot)? Does he stay well-hydrated?

I don't think we can really help. He needs to see a doctor to figure this out. Whatever is causing these symptoms, they aren't normal and he needs an answer as to why they are happening. I hope you can figure that out with his doctors.

Annie