While it is often difficult to determine whether problems in adolescents are transient manifestations of the developmental process versus symptoms of psychiatric illness, some general points can be made. I recommend that parents look for two potential indicators: distress and dysfunction. Either one can suggest a psychiatric disorder.
The first is simply the subjective experience of being ill at ease or unwell - e.g., excessive sadness or nervousness. The latter has to do with a failure to perform the age-appropriate tasks expected of individuals. For most adults, these age-appropriate tasks would include functioning well at work, taking care of a family, maintaining responsibilities within the community, etc. In the case of adolescents, these would include maintaining an expected level of academic functioning, following household rules or performing chores, adhering to behavioral standards within the community, etc. You mention that your son is not doing well in school. Assuming you mean that his academic performance has declined recently, this might be an indicator of a psychiatric problem.
In order to pursue this further, I would recommend first discussing with your son his perspective on the changes that you observe - especially his academic decline. If you and he can identify a particular factor (or group of factors) that would explain his school difficulties, then these should be remedied. If you, he, and school personnel are unable to help him bring his academic performance back to the expected level, then I would recommend consulting with his primary care physician about the advisability of a psychiatric evaluation.