I had to get my daughter's attention when she was a teenager. Sweet girl away from home, but she reserved all of her strong will for us at home. It was exhausting and I just couldn't take it anymore.
While she was at school, I packed up her whole room (except for clothes) and put the stuff in large trash bags. She was really into decorating her room and had cute bedding, etc. I took everything off the walls, replaced her bedding with old off-white, dated stuff, took her stereo, CD player, etc. (that was all the electronic stuff she had at the time). Her phone was also taken from her. She needed to learn that nothing really belonged to her as we were supporting her.
I told her very calmly when I picked her up that afternoon that she wasn't going to like what she saw when she got home. But if she raised her voice, slammed a door, etc., it would get worse. I just simply told her that she had not been acting appropriately and she was going to have to earn everything back over weeks of time.
She was shocked but calmly asked where everything was. I told her where and that I left her clothes. If she accepted things as they were, responded politely, etc. then I would leave her clothes. Otherwise I would leave only white t-shirts and plain jeans for her to wear everyday.
Life changed drastically for the better from that day on. She had small outbursts from time to time but they were few and far between. She had learned to control herself and speak calmly and politely when she disagreed with something. To this day, years and years later, she will tell you it was the best thing we ever did to get her attention.
Keep your son's things for several weeks, not days and let him earn back the stuff. He probably has a lot of endurance if he is a strong-willed child.
Way to go in checking his phone. As a teacher, I was so sick of kids sleepy in class because they were texting all night... Parents need to wake up! If everyone in their class has a phone, then they really don't need one. They could borrow one in an emergency. Just get the numbers of their friends so when they go hang out (if not grounded), you can get a hold of them.