J.,
I work with Crisis Pregnancy Centers throughout the country and do public speaking, so I work with teenagers on a regular basis. I see this all the time. Most of the simple steps I could tell you to start off with you've probably already tried so I'll take it to a deeper level.
Assuming you've talked to your son about Sex, girls and all that, take your son to a Crisis Pregnancy Center, most of them have male counselors there, they'll talk to him about what they see every day, give him some information about STD's, about how to find the right girl for him, about how to judge a girls character, etc. They may be able to offer some information to you as well. You may have tried talking to him about such thing but teenagers, especially boys don't want to hear that from their parents. It might sink in if he hears it from someone else. He's got alot of emotions, adrenaline, and hormones trying to take over right now and he doesn't know what to do with himself, thus all the activities. But when he's home, alone just you and him that's when he needs to know what to do with himself.
If you are a religious person I suggest the care net centers rather than planned parenthood, but the choice is yours. You can find one near you by going to www.carenet.com
It might not seem that this is the source of his issues but girls are at the forfront of his mind right now. Especially with what you said about his cell phone. Girls this age are confused and far more racy then we ever were. My own daughters are teenagers and it still amazes me what the girls in their school will do. Most of these girls don't know who they are nor do they show any respect for themselves so of course they won't show respect for your son. He must learn how to respect himself and them as well even when they don't respect themselves. NOT an easy thing for a boy to do, hey his reputation is on the line (at least that's how he'll see it).
He's still young and he's got time but if he doesn't set boundaries for himself now and decide for himself what his values are in regards to his future and girls, before he's put in a compromising position then when it comes up he may make the wrong choice. But the key factor for success is that he must set the boundaries, he must make the choice, he needs to feel empowered to do that and knowledge is power.
My book "Surviving the teenage Journey" was written mostly for teen girls but perhaps it will give him some insight into how they're thinking and why they are doing the things he sees them doing. I've had a few responses from parents and grandparents who said that after having their daughters read my book they were going to have their sons and grandsons read it. There are quite a few books out there and if your son is willing to read one maybe you could find one to fit his personality, that would be helpful for him too.
Good Luck! Hope this helps...