I don't think so, in the books Robert has many many bastards, but in the show I think he's the oldest of Robert's kids we've seen. Also, Joffrey isn't Robert's son, so that doesn't really work either.

You're forgetting about the scene with Loras and Littlefinger's spy earlier this season. And this episode even showed part of Gendry's penis, which I'm sure thrilled a few people.