Then give him the responsibility for following through on the plan. Talk to him about his future goals. He may talk about what he wants, like a fancy car or he may talk about wanting to be a doctor or electrician. Whatever he mentions, you can then ask him how he plans to make that happen. No matter what his goals or dreams, finishing high school is a necessary step.
Dreams are great motivators.
This way, you're letting him know that you expect him to do his best, but giving him control over when and how. And you're right that the girlfriend is a distraction. Encourage him to invite his friends to your home. Make them welcome to let him know you are encouraging his social life. Get involved with your child's school.