I would tell him that other foods give you vitamin A other than carrots and he probably gets the recommended amount of vitamin A without knowing it, or he could just eat another carrot if he's that worried about it.
It's very easy for things to go awry when there is nothing documenting the agreement between the parties or variations to that agreement. That's why it's important to ensure that any agreement you have, whether with an employee, supplier, customer or business partner, is captured in writing and any variations to the original agreement are also documented, approved and signed off
