Thank you for your question. It sounds like you are concerned about your son and I can certainly appreciate why. Depression can be diagnosed a number of ways. However, it is my understanding that life coaches are not qualified nor should they make any formal diagnosis, only a licensed mental health professional.
Most often depression is diagnosed by a comprehensive clinical evaluation. A licensed mental health professional evaluates the individual by asking him a series of questions that include identifying if depressed symptoms are present and if he meets the criteria for depression along with a life history, medical history, and many other pertinent areas. There are formal tests for depression which are often used to support the information obtained in the evaluation but it is not required to make the diagnosis. What is important is to make an appointment with someone who has experience with assessing and treating depression.
The same holds true for ADD, although its more common to use formal testing to diagnose ADD than depression. Nevertheless, a good, thorough evaluation can often provide enough clinical information for a trained professional to make a diagnosis. The formal test is, again, used to validate the clinical findings of the evaluation or answer a question.
Formal testing is expensive and time consuming. Insurance companies may pay for them but it depends on your insurance plan. Evaluations can also be costly but not as expensive.
Your son should find an expect in depression or mood disorders or ADD (depending on which one is causing the most distress) in your community and schedule an evaluation.
Its important to note that If you are depressed you can have trouble concentrating which makes people wonder if they have ADD but if you have ADD and its making it hard to manage your life you could feel depressed. For that reason, its important to get a comprehensive evaluation.
I hope this information was helpful. Good luck to you and your son.