How can we encourage our teenager to work harder in school when he's not a great student?
I understand your problem. My son also was not a great student. The trick is to discover what his passions are. He may be more a hands-on kind of guy, and becoming involved in an internship program in the trades may be perfect for him right now. Heavy-machine operators can make $70,000 to $80,000 a year, and that would be enough money to support himself and for him to pay for college later on, if and when he decides to attend.
Pushing our teens into college is kind of like trying to push water uphill. It just doesn't work. It will cost you money, frustration and heartache, and he probably will fail. If that happens, he will feel like he not only has failed himself, but you, as well.
Spend some time with him and find out what he wants to do. Sometimes it is important to listen to our kids as a job coach instead of as a parent. You could suggest that he take one of the many occupation preference tests. He might discover something incredible that really excites him that you never thought of. By the way, my son never went to college, but now owns his own real estate company in Hawaii.