A 32-year-old man with schizophrenia has been on risperidone 6 mg/day for 8 months. He presents with complaints of progressive difficulty in initiating movements, muscle stiffness, and tremor at rest. His wife notes he has become emotionally withdrawn and apathetic over the past 3 months. On examination, he has cogwheel rigidity in both upper limbs, resting tremor, and bradykinesia. His temperature is 37.2°C, and creatine kinase is 245 U/L. What is the most likely diagnosis?