What was the last thing Sohrab said for almost a year? Why?

"I am so khasta". (I am so tired)

Although, I do not know the answer to your question, we can infer that Sohrab has descended into a place of hopelessness. He misses his father and mother, he has experienced inexplicable atrocities, and he doesn't quite trust Amir...... he's afraid of being abandoned.