Jessica, it's way past time for you to grow a SPINE and put your foot down, and give your son some TOUGH LOVE, which he badly NEEDS right now. Start by getting rid of all the electronics. And that means EVERYTHING, except perhaps for the computer he needs for school, and even that must be kept in a public place, such as the kitchen or family room. Get rid of your TV, DVD player, your Wii set if you have one, and every other electronic device you own or that your son has access to. If he's got a cellphone ( which he shouldn't have, not at his age) get rid of THAT, too. Keep your OWN cellphone in a LOCKED, inaccessible place when you're at home with him.
Once the electronics are gone, you need to have a discussion with your son in which you make it clear what the house rules are. At your son's age, he should be getting at LEAST 10 hours of sleep EVERY NIGHT, in order to remain healthy. This is what research has shown that teenagers this age need. That means you need to set a BEDTIME of no later than 10PM on school nights, and 11PM on the weekends. And bedtime means just THAT, he goes TO BED, without watching TV, listening to music, or doing anything else that is stimulating. If he needs to hear something in order to fall asleep, buy a fan or a white noise generator, and put that in his room, which incidentally needs to be very DARK. The idea is to get him to fall asleep from boredom, if for no other reason And he will do that, if the circumstances are right. But under NO CIRCUMSTANCES should he be permitted to have access to electronics of ANY KIND, period. The only exception is the tablet/computer he needs for school. And in that case, it is YOUR JOB as a PARENT to watch and supervise him while he's doing schoolwork. You need to make an additional house rule that all homework must be completed by no later than 9:30PM each school night. That will give your son half an hour to prepare for and get in bed. Lights out will be at 10PM.
You should be prepared for rebellion and blowback from this, but it's important that you stick to your guns on it. Your son is going to be mighty pissed off, especially at first, but eventually, he will settle down once he finally gets the message that you're serious about him adopting healthy sleep and work habits. And you can help him find ways to EARN the right to have access to things like TV and music again. You can even use that in place of an allowance, in fact, or as a reward for his doing extra chores around the house. He's more than old enough now to learn how to do things like empty a dishwasher, run a vacuum sweeper, take out the trash, etc., on his own, and his doing so will make life easier for you.
Lastly, your son would benefit enormously by becoming involved in something like sports, or perhaps going into scouting. If he's busy with these kinds of healthy activities after school, he won't have as much time to be on the computer, and more to the point, he'll be tired enough to sleep at night. Many a teenage boy's problems have been solved by being involved with sports, music, theater, or some other activity that gets him out of the house, off the computer, and doing something constructive when he's not in school.