I'm Regressing

I just read Diary of a Wimpy Kid by Jeff Kinney. I think it was written for middle school boys, but it ended up in my library bag somehow. It was short and cute and I'd love to recommend it to my nearly 13 year old nephew. Since I don't have children that age anymore, I'm not sure it's appropriate. The main character was kind of a smart mouth and I don't want to give my nephew ideas (or make my sister angry with me). He's read all the Harry Potter books, so maybe he can handle it? Ideas, please!