How to help my son find motivation?

My son is a senior in high school. His grades are pretty bad. Teachers say he doesn't try enough, doesn't turn homework on time. We tried everything - therapy, diagnosing a learning disability (none found). He told me that he will go to community college next year and therefore we should leave him alone. He supposedly is looking for a job, but in my mind not aggressively enough. Spends a lot of time at the computer, although we tried to limit his time. I would like some feedback from mothers who were in the same situation and their children improved with time. Otherwise he is polite, helps at home, cares for his siblings, does not abuse curfew, doesn't smoke or stay very late with friends partying, etc. He just seems unmotivated to do anything society would consider meaningful - like studying, reading books or working. Am I too demanding? What really works in this situation?