Adolescence is a time of great change as teenagers are starting to develop the skills of making their own choices and managing their own lives in preparation for adulthood--even though they aren't quite there yet. During this time of developing physical, emotional, and often sexual independence, there can be a great many stressors that can be very difficult for some teenagers to handle. As you have noticed, some teens may not want to talk about these challenges with their parents, even when something does seem to be bothering them.
It is great that you have a close enough relationship with your son that you are noticing something seems to be going on. It sounds like he may be hesitant to talk with you about things that may be bothering him, but that doesn't mean you should stop letting him know that you are there for him should be want to reach out. It is also a good idea to make sure that he has regular annual visits with his pediatrician
, as he may be more comfortable bringing up particular issues in private and a physician who knows him well may be able to screen for depression
. There are definitely therapists
who do specialize in working with adolescents, but without knowing exactly what is going on with your son, it is hard to know whether this would be appropriate. As a concerned parent, in addition to speaking with your son himself, you can also talk with his teachers at school or other adults he may interact with (parents of friends, coaches, etc) to see if they have noticed a change.