When you start your turn using Stealth, you can leave cover or concealment and remain unobserved as long as you succeed at a Stealth check and end your turn in cover or concealment.
The text seems pretty clear to me. Yet, there is no mention of what happens afrer your turn when monster walks around the cover.
Goblin rogue in my campaign have +24 stealth on check, and its kind of annoing that even if he is surrounded by monsters, if he stands next to a street pole, he can just make 5-foot step to hide. Even at -10 penalty for doing it in combat, he is pretty good at it.Still, it looks kinda cheesy, even if he is surrounded, or monster walk around the cover, rogue remains invisible. Am i missing some rule?
Cover is only used to make the roll, not to maintain stealth.
However, make sure that he really has cover. If there are multiple opponents, chances are good that (particularly behind a lightpole) he doesn't have cover from everyone. Force him to make a bluff check (Goblins generally aren't so good at that).
Similarly, if that's his schtick... let him run with it. He obviously put a lot of effort towards this thing, and it's ok that a character remains unseen most of the time. It's also kinda the rogue thing... I mean if you read the D&D comics, the rogue girly, she always disappears entirely before a fight... that's her thing. It's only something to buckle down on if it's game-breaking.