Explain in terms of common sense. Make sure your teenager understands that a clean bathroom is more pleasant to use and minimizes the spread of germs. Encourage him to help you in cleaning the bathroom.
#2: Bounty hunter technique
Offer a reward for cleaning the bathroom, such as cooking your teenager's favourite meal, etc.
Step 3: Merciless mommy method
Stop cleaning the bathroom altogether. After a while, your teenager will clean the bathroom on his own accord because he will get tired of the mess. Extinguish any urge to clean it yourself.
Creatively inform your teenager about how much you go through in cleaning the house, doing the laundry, cooking, washing wares, etc. Challenge your teenager to agree that the very least he could do would be to clean the bathroom.