Great news Esperanca... . that your son has set such a goal for himself.
I think it is a good thing to expess authentic concern for him... . not if he will pass this examine... . concern for how he will handle the stress and pressure of studying for this exam. If he studies hard and stays on task he can pass the exam... . yes?
It is important that he knows you believe he can do it... .
Perhaps asking him some validating questions can help him think if he is ready for so much work and can stay focused for the long time it will take to be prepared to pass this test.
Example:
Ds, I am so glad that you have this goal for yourself. I think you will make a very good
Policy Agent. Are you prepared for the long hours of studying to pass this test? Have you thought about how I can help if you have trouble focusing?
I hope that he will believe in himself and achieve his goal! To succeed at such a goal would help lift his self esteem and that is good.