Would you force a teenager with cancer to undergo chemotherapy against his will?

Work with counselor. Before ~25y the brain is still developing & decisions lack some of what we accept as adult level competance. The 16-17yo is 75% there, but quite capable of understanding much of the issues. At the same time many have unrealistic fatalism or invulnerability ideas.Each kid & situation is unique. Confer with the oncology team, counselors, psychologists. In the end the parent must make a decision.
Yes. Many cancers of young patients are cureable. What a trajedy if this teenager is refusing a life saving treatment. I would rather convince him with literature and data that what is being recommended is necessary. As a parent i would want to save my child's life.
It depends. This depends on many factors. Some types of cancer respond well to chemotherapy and some poorly so I would more strongly encourage it if it seems likely to help. Some teens are more mature and able to make a major decision than others, so I would consider that. If the teen refuses chemo he or she should consider alternative therapies; taking supplements can also minimize side effects of chemo.