First he should write out his negative thoughts (most negative thoughts are repetitive).
Most of them are also untrue. He should write that the thought is not true. Then he should write a positive thought.
For example, suppose he thinks he is stupid. He should write the down, then write why he is average intelligence (or smart or whatever). Then craft a positive. That needs to not use the word not. So, he should not write, “I am not stupid.” but something like: “I have the brains to do whatever I need to do.”
Each time he catches himself thinking the negative thoughts, he must stop himself. He must replace the thought with the positive thought. If possible, he shuld say it out loud.
This will feel awkward at first. He has to persevere.
Good luck to him.