How to express that a person is not suitable for work?

If you are in charge, you can say so. Actually, you did a good job. You can list the risks of this person, and then talk about shortcomings, such as what skills our foreman needs here, or how to face customers. It's best to pick what he obviously doesn't have, so you don't say whether he is suitable or not. Can you say that you think you have all this? I decided to keep it for myself, so as not to make him feel lost. Finally, let's comfort him and say that you are still quite capable and practical. I believe that if you continue to work hard, you will do well. This will give him some motivation and keep people.