If you have a teenager who has too many activities to do or begins to have too many responsibilities, this is not good or productive for him / her. If you have activities or responsibilities, that's fine ... but having too much is not good for anyone. If your teenager is too busy, you should not sit idly by.
The first step is to sit your teenager down to see how he feels. There could be a rare case where your teen just wants to do all the activities! In other cases, you might protest by dropping an activity or two, but it may be necessary to slow down for your well-being.
You will have to make the executive decision to remove him from an activity of his choice, as long as it is not homework or study! You can always get it back in the future if you find out that you really like it and want to go back to it.
It is important to keep in mind that the teenage years are a vital time for your child to learn to manage their time. If he wants to find time to do everything, he will need you to come in and help him say no to certain activities. Similarly, you will need to make sure that you don't put too much pressure on him to get things done. It is not worth enrolling you in a very important language school if you are so exhausted that you cannot function when you get there.
If your teenager is really struggling to cope with the usual and normal demands of life for his age, then ideally, seek professional help. Whether you're a perfectionist, procrastinator, or overkill, a mental health professional can help you develop healthier habits.
Finally, you should keep in mind that some teens, like many adults, see being busy as a symbol of status or social need. Make sure your teen knows that his self-esteem doesn't have to depend on how busy he is or what his social calendar is like.