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Rude as in how?
If you mean rude as in "he makes fun of me harshly" or he makes fun of something that I have trouble with/can't control (i. e.; weight, disorders, et cetera) then he's just being... well.. pretty much a bully.
There's also the possibility, though, that he's just trying to tease you lightly/flirtatiously, but doesn't realize it's coming off as rude.
But, then again, there's also the chance that he's just blunt and doesn't realize that what he's saying is rude.
It's hard to say for sure without examples or having been there your/myself to hear the tone he had. (Tones are fairly crucial, in my opinion.)