You need to quit complaining about how much of a fuck up your kid is, because really, a lot of the blame should be put on you. He is why he is because you failed to shape him better by letting him slide.
Right now, school is the most important thing for him.
First off, take away his computer for anything other than school work for a couple weeks. Keep him home and make him study for at least 2 hours a day or whatever. Ask his teachers for grade reports every week or two, and have them E-Mail the grades to you so your son can't give you bullshit excuses like he forgot to bring his grades home.
Don't let him go out with friends if his homework isn't done. School comes first, make him know that. His education will take him further than football or working or bullshitting with friends
You need to get involved in his education. Help him with his homework, help him study and ask to look over his tests and things. Make him feel like you care about his success, and take a hand in helping him improve. Get him a tutor if need be, just make sure YOU are making and effort to assure his grades improve. Get him looking at colleges too, and have him apply for colleges and scholarships.
Don't take football away, since his grades aren't stellar, that might help him get into college. It also helps make sure he has a commitment and will help him develop responsibility.
I agree with other people here that he should get a job. Stop giving him money to go out and tell him that if he wants to go out, he has to come up with his own money. He will learn responsibility through a job as well as the value of a dollar.
Ground him when he fucks up. Don't let him go out on the weekend, and during the week only allow him to go to school, football and work, etc. Don't be lax with your punishments. He isn't going to punish himself and turn over his computer to you or stay home on grounding, etc. YOU HAVE TO TAKE RESPONSIBILITY AS A PARENT AND FOLLOW THROUGH WITH YOUR DISCIPLINE!!!!
and last of all, if need be, whoop his *** or kick him out of the house for a night and see how that works. tough love is the best love sometimes, give him a dose of reality.
and btw, i'm 19 and responsible, I'm in college making good grades, stay out of trouble and work 2 jobs. my parents had me work, took initiative to make sure i was doing well in school, and disciplined me when i needed it. Thats what you need to do for your kid, he'll appreciate it later, even if you guys fight about it now