What Causes Constant Headaches And Stiffness In Neck Despite Taking Prednisone?

my son is 32 years old. on short term of prednizone for poison ivy. received an injection f.irst and then 20 mg prednizone i believe for 1 week. Began getting severe headache and no relief from poison ivy. Dr. pres. another week pred. said headach would go away. Severe for about a month after and still has slight headache and neck stiffness. Happened 6 months ago and still not back to work. Sleep has been affected and his anxiety has been quite high with depression. Is on 75 mg. of Zoloft and ? amount of Welbutrin. Has lost weight. Any suggestions and what type of doctor may be best to go to? Thank you. YYYY@YYYY