He said something to that effect at one point, that Rolling Rock was just whatever beer he had around, so he decided to make it a thing. He eventually got a Rolling Rock clock that he's shown a few times, too.
I figure he's probably beloved at Rolling Rock HQ, and probably gets all sorts of free merch and factory tours and drinks for free in place of any kind of legally-binding advertising contract. I always figured if I were he, I'd just design and print up increasingly goofy beer labels every episode, and just put those over whatever bottles I'm using.
