It definitely sounds like your son wants to have a conversation with you. There will probably be several parts to the conversation. So, to address one of your questions: is your son gay? The answer to this is probably yes—and no. You have to understand what it means to be gay and also understand the term “bisexual”. While not universally true, most people who consider themselves gay feel that they are only attracted to people of the same sex, not people of the opposite sex.
In identifying himself to you as “bisexual” in his email, he is saying that he has feelings (sexual and/or emotional) for people of the same sex, but that he is also genuinely interested and attracted to women as well. A person who is bisexual does not have to date more than one person at a time, but is open to the possibility of loving someone of either gender.
Your son has dated heterosexually in the past and may or may not have also had feelings for male friends or experimented with sexually with other boys. Most boys who do so do not tell their parents. Of course, it is much more acceptable to admit one’s heterosexual feelings than to admit same-sex attraction so it is really hard to know if the feelings he has for men are newer feelings or feelings he has had for some time.
Often, a bisexual man or woman does not feel the need or desire to come out until they find themselves forming a significant relationship with someone of the same sex that he (or she) is really excited about. People come out in all kinds of ways these days. While Facebook might seem like an odd place to announce one’s sexual orientation or introduce friends to a new dating partner, it is actually quite common now.
Given your son’s email to you and his postings, it should be fairly easy to start the conversation. I would suggest bringing the subject up in a return email, phone call, or personal visit (if one is happening fairly soon). He may initiate the conversation himself, but if he doesn’t you might ask him to tell you about his new friend–or about his new sexual identity realization.
Either way, I think you will you will find that he is willing to talk to you about this new development in his life, especially if you show genuine interest and are non-judgmental. It may be a bit awkward at first, but feel free to ask questions. It may be an unusual conversation to have with your son and a bit of a difficult one, but it is a conversation well worth having.