The first and most common is a normal human attitude that more is better. It could be that his problem has a social basis. Any person from a particular group of people such as athletes are only truly understood by that group of people. Being an athlete, it is also possible that the only way he knows how to recreate and truly enjoys recreating is to workout. Therefore, if he has extra time on his hands, he works out.
Many athletes only know how to or only enjoy recreating with sports.
The workouts the day before the race could be a result of nerves or insecurity. He may get stressed out about tomorrow's event and worry about whether or not he will be ready. This can be a very complex problem. The very first thing you must do as a coach is to get him to explain his TRUE inner feelings about why he is doing these things. Then you can decide what to do to solve the problem.