Ok, thank you for the additional information. This is actually very common in teenage boys (up to half of them have it to some degree) and the medical term is "gynecomastia." It is usually due to hormonal fluctuations in adolescence. It can also be due to obesity but this doesn't sound like it's the case with your son. Although it can be embarrassing, there are usually no medical therapies that need to be undertaken. There is more information available to you here.(written by a pediatric endocrinologist)
Clearly if there are other concerns about pubertal development (which again do not sound to be the case), then he should be thoroughly evaluated.
I hope this is helpful and reassuring to you (not sure if it will be reassuring to your son, but either is acne). If there is anything else I can help with, please do let me know. Best wishes to him and to you. Thank you, XXXXX XXXXX