[andrei_says_]

Track your tasks and ask him if the new task has the highest priority. If sincerity is an option, inform him of the cost of interruptions when someone is doing deep work. Propose a system for following up on and completing tasks.

[codingstuffs]

I like the idea of asking if the new task is top priority.

At the end of the day what he says goes, so if it really is what he wants, better to make it clear, and then say, ok, I will then prioritize the new Task B instead of Task A and set expectations.