Somebody would ask for the specifics (whos the step-parent etc) but I would consider he's still your family. I'm not into making judgments as people are different depending on parts of the world. I am also curious as to where you live and is that what your family have told you?

Okay so if I say your grandma have a son and a daughter with 2 different men, the offspring are considered half-siblings. Which is the case with you. If your mom (who has your grandma's bloodline) had you with a different man, they had you. If his dad(with your gradma's bloodline) married a woman and had him, he has your common grandma's bloodline.

You are half-cousins, blood related.