The best way to explain climate change to your teenager is to let him know that there is strong evidence that the average surface temperature of the Earth has warmed by about 1.5C, ever since the thermometer was invented. That's not much, but it is enough to have caused the sea level to rise by about 8 inches (mostly because warmer water expands).
We know this warming is due to increased carbon dioxide in the atmosphere, mostly from burning fossil fuels. That's called the "Greenhouse effect". Carbon dioxide increases the effectiveness of the atmosphere at trapping heat, so when carbon dioxide increases, it gets warmer.
Also tell him that he will hear a lot of exaggerations from people who are genuinely worried about future temperature rise. We do expect that the temperature will rise another 1.5°C in the next 50 years, from continued carbon dioxide releases predominantly from China, India, and the developing world. They need more energy to improve the standard of living of their people, so we need to help them produce energy with low carbon emissions. That means nuclear, energy conservation, and natural gas.
He should know that the 1.5 C rise in 250 years is too slow a rate to be noticed by any humans. People who claim that they can see climate change are actually disagreeing with science, which measures the change as quite small, so far. But another 1.5°C rise in 50 years could cause hardship.
Sea level is expected, in that period, to rise another 4 to 12 inches. That won't be enough to have disastrous consequences (not even in Bangladesh), but some alarmists claim (incorrectly) that a 4 to 8 foot rise is likely. They are exaggerating, possibly out of conviction, but they are not accepting the science of climate change.
There are many exaggerated claims, about increased storms, floods, and droughts, but none of them are scientifically established. Indeed, they are not even universal predictions of the models; some models predict increased storms, others predict decreases, and others predict no change. Climate change has many alarmists and exaggerators. He should watch out for them and ask them for the evidence. Then he should check with people who disagree (Quora is a good way to do that) and find out if the "facts" he was presented are true. Doing this will be good practice for evaluating facts in everything else he encounters in his life. (I recall, in 6th grade, when my sister showed me that I should not necessarily trust advertisements!)
There is no need to panic over climate change, but there is value in acting. Ironically, our next president, Donald Trump, has expressed skepticism over whether the world is really warming or not. But his beliefs hardly matter; if he supports additional nuclear power, natural gas, and energy conservation (and he clearly supports at least the first two) then his actions might actually be effective in slowing global warming.
He also needs to beware of the strong convictions of his fellow students and possibly those of teachers, who may be passing on what others have told them without carefully checking the facts. Some people believe that any questioning of facts of climate change suggests that you are anti-science. Indeed, it is the opposite that is true. Those who tell you to accept authority, and not be skeptical about what you are told, are the ones who are acting against the principles of science.
Global warming offers not only a lesson in physics and geology, but also a lesson in human behavior.