Step 1: Understand the sentence.
The sentence "He is worried in his father's illness" suggests that the preposition "in" is incorrect in this context.
Step 2: Identify the mistake.
The verb "worried" is usually followed by "about," not "in," when referring to a person's concern over something. The correct phrase should be "worried about."
Step 3: Conclusion.
Thus, "in" is the incorrect word in this sentence.
Final Answer: \[ \boxed{\text{The incorrect word is 'in'.}} \]