How do I tell him that I don't find joking around with my looks funny?

Anonymous
So I know this guy who I see from time to time because he is in the same friend circle as my boyfriend, and I don't particularly like this guy. He's incredibly immature, and I have a feeling he finds me a little too uptight. For some reason he's made it a thing to call me fat or obese as a joke (I'm actually underweight), but I don't find it funny and I don't react as if I do. I had an eating disorder for over 10 years, and it never really goes away, it's just sort of dormant right now, but I know it can come back. He of course doesn't know this, but he certainly should be able to tell that I don't find it funny, since I have not once laughed.
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He also often calls my boyfriend "skinny", and my boyfriend isn't bothered by it, because he is pretty thin, but I've always been afraid of being "bigger" than him, which again I'm not (at least I don't think so, I can't tell because of body dysmorphia though), but this guy saying that just feels like another jab at me.
How do I tell him that I don't find joking around with my looks funny?
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