Is your company supporting your use of ChatGPT4 for research? If yes, you could respond to him that you appreciate his concern and you understand where he’s coming from (many people and companies oppose to or in favor of seriously limiting use of AI for research) but since company doesn’t object it you’ll continue using it.

Also it seems that agency doesn’t care if he’s bugging you. You said you complained about him but here he keeps doing it. So if they don’t do anything about it, you have to figure out how not to let it stress you. Thank him for his concern and tell him you’ll look into it. Then do what you think is right.

If agency or your boss don’t like your work, they’d let you know. Him not liking it is irrelevant. That’s just his individual and subjective opinion.