If I recall, it was a childhood nickname. I don't know for sure, but maybe his dad called him porkchop or something one day and it got shortened.
When I was in my early 20s, I had a girlfriend I would occasionally call lambchop as a nickname. (Don't know why or where I got it). I'd been using it for a while when one day, out of the blue, I called her porkchop instead of lambchop. When the realization of what I had said registered, I froze like a deer in headlights. Dare I say, she was NOT happy and it took quite a while to convince her it was an accident and that no, I don't think she is fat. Behind the scenes, it was funny as hell and still makes me laugh.