I'm going to give my boyfriend and ultimatum.
What has led you to think of setting an ultimatum?
I want him to stop eating my food and buy his own.
This must be very frustrating for you and I understand why you would want to give him an ultimatum. He should be more considerate of your needs and what belongs to you. Have a discussion with him so that he understands how serious this is for you.
I made that ultimatum as a joke. I'm going to make him a sandwich before he goes on his long drive tonight.
Okay, I'm glad to hear that you were just having fun.  It's good that you are supportive of your boyfriend's needs, and I hope that he does the same for you whenever you need help.