Another guy likes my girlfriend, what should I do?

Me and my girlfriend have been together for over 6 months, and I know we both love each other. This guy has been texting my girlfriend and her friend for awhile now, and I didn't mind it until the texts he started sending started getting sexual. He and my girlfriend's friend, let's call her Bob, have started completely sexting. Bob and him are getting really sexual and that's one of the problems, she has a boyfriend already, and Bob is good friends with my girlfriend so Bob included my girlfriend in these texts. I used to be great friends with this guy but now we're just classmates. So after these texts, Bob and my girlfriend now know his penis length, when he masturbates, and who he thinks is attractive. Bob and my girlfriend are both on the list. I'm certain that he likes my girlfriend, considering some of his sexual facebook posts. He acts as if he is completely oblivious to my relationship with her, but we talked about it earlier in the year, so I know he knows. This guy is VERY cocky, and he thinks he can get anything because he's attractive (I'm not gay, but he is appealing to females). I know my girlfriend won't cheat on me, but there's some things that bother me. First off, Bob, my girlfriend, and I were all hanging out at Bob's house and they both started texting him, did i mention my girlfriend texts him and Bob more than she texts me? Anywho, he told them his penis size, and according to him it's 8 inches, but that doesn't matter, what matters is the reactions. Bob was amazed, but so was my girlfriend. I'm going to be honest, mine is only 5 inches and I'm proud of it, but hearing this made me feel a little sad. My girlfriend immediately went to get a ruler and measured out 8 inches, which made me even more uncomfortable, considering I was right beside her and she was breathless. Then he told them about how he masturbates, which really bothered me because both of these girls were paying full attention. I wasn't included in any of these texts by the way. So basically I just sat there while my friends gave this guy their undivided attention. That really made me feel like ****, pardon my language. That was about a week ago though, but this week it was basically the same thing, except he told them who he thinks is hot in our school. He said a bunch of ladies I know, a few I didn't, and then he said "and my bitches". Again, sorry for the language. After a while of listening to them, i discovered that both Bob and my girlfriend were his bitches. That made me feel really sad. Not the fact that he called them bitches, but the fact that they accepted the name and were kind of joyful. Throughout the rest of my visit there, I was noticeably depressed. My girlfriend asked me what was wrong and I lied to her, I know I shouldn't have done that, but I really couldn't think of what to say about it. So basically what my question is what should I do about this guy, how I should confront my girlfriend about it without pissing her off, and how to possibly get him to stop sending this nonsense. Sorry if this was too long, but I really need answers. I don't know what to do, I'm really worried and sad. I know my girlfriend won't leave me for him, but that scenario just keeps on playing in my head. I just want this to stop :( . Tomorrow, I'm going to ask my good friend about it and get his opinion as well. Again, I apologize if my argument was too long. I tried to use proper grammar and such. I'm 15 years old, and in grade 9 (just so you get a better visualization). I hope you guys can help me :) . I thank you in advance. Please, serious answers. :)