He needs an action to take to solve his problems. You say he's good at fixing cars and things. To fix a car is to take action. Maybe you should try to figure out what actions he could take to build a better attitude. So, helping him to have a better attitude means helping him to become more optimistic.
People can learn to be more optimistic.
There is a short test near the front of the book. Maybe you could read it aloud to him and have him answer it. It has questions about what would you do in this situation or that. They aren't hard questions, they are just to get an idea of how people would react in certain situations. At the end is a way of finding out where people are strong and weak in their attitude.