I'm happy that you have asked me to assist you with your child and hope that you can benefit from my many years of helping parents like you. This is quite normal as boys and girls enter puberty. Your pediatrician is correct in telling you that lots of boys have some breast enlargement, though really only in about 25-30% of boys. This is not due to estrogen production either but rather due to the large outpouring of testosterone at this time in his life and the fact that one of the metabolic breakdown products of testosterone as it is "recycled" it an estrogen-like hormone that can cause some breast tissue enlargement.
Okay thanks for that but where are the feelings/attractions for boys coming from?
That said, estrogen has nothing to do with being gay and the jury is still out on whether "nature or nurture" are the underlying influences on attraction to the same sex. It is quite normal for boys at this age to have "boy crushes" just as girls do as this is a tumultuous time in their lives from both a hormonal and social standpoint. All you need do is reassure him that these feelings are developed over time and normal for all boys and girls his age. As his testosterone exerts more control over his body changes with puberty and socialization begins to define his role then I'm sure his feelings will be clarified. That is not to say that he definitely will not be attracted to other guys but the majority of boys go through these periods in their pubertal development and most develop the opposite sex attraction that parents of teen suffer through eventually.
You are fortunate that he is willing to voice these concerns to you as most boys do not and that leaves them even more confused and prone to experimentation with other boys, which by the way, is much more common than most people realize. Just reassure him that these feelings are normal in all pubertal teens and that you are always available to discuss this with him.
I have to go see a few patients but I'll check back to see if you have further questions later. If you decide to accept my answer you may still return with further questions at no additional charge.
Here is a reference on gynecomastia in boys:
Thank you very much. Generally how long does it take to go through this phase? Should we take him to see a counselor at this point?
Assuming he follows the normal puberty stages the gynecomastia last a couple of years and by then most of the hormone surge is over. I would think though that if he is really bothered by his feelings (worried about his sexual orientation) counseling even now might be worthwhile. Be careful to choose a good counselor who is non-judgmental and unbiased, perhaps a Christian counselor if that fits your family beliefs, and his doctor can probably recommend someone they are experienced with.