I'm going to tell a 16 year old that he won't amount to anything
Why do you want to discourage a 16 year old?
Because I believe it.
Instead of focusing on the wrong things in thier life, you should focus on what they are doing right. Encouragement will get your point across much quicker than criticism.
I was wrong. I should encourage him.
I'm glad you have changed your mind, you should always be encouraging your own children.