Hi, I'd like to help you with your question.
Sometimes younger people need the time to figure out what they want to do with themselves. They often work minimum wage jobs just to get by until they find their way. There is nothing wrong with this as long as long as he is supporting himself and not relying on others to get him by and he is not showing signs of a mental health disorder.
The only other concern is whether or not his depression is affecting his ability to be motivated and to what extend. If you see that he still has symptoms of depression and it affects him in other ways, such as laying in bed all day, not caring for himself, crying a lot, appetite changes or any suicidal thoughts, then you should encourage him to get help. He can see his regular doctor and take a mild anti depressant until he can get back into treatment and deal with the depression. Lack of motivation can be a part of depression, but if you do not see any other symptoms, then he is probably fine.
There are also some books that may help you talk with him. One is called Talking to Depression: Simple Ways To Connect When Someone In Your Life Is Depressed by Claudia J. Strauss and Martha Manning. Another is Could Do Better": Why Children Underachieve and What to Do About It by Harvey P. Mandel and Sander I. Marcus. Also, this book may help as well Boys Adrift: The Five Factors Driving the Growing Epidemic of Unmotivated Boys and Underachieving Young Men by Leonard Sax. You can find these on Amazon.com or your local library may have them.
I hope this has helped,