A:  It appears your boyfriend experienced a classic case of jealousy.  Jealousy arises out of insecurity.  Since you knew this guy from high school, chances are your body language and conversation with him were very comfortable, animated and friendly.  Seeing you have this level of camaraderie with another man probably made your boyfriend feel threatened.  Rather than rationally dealing with his emotions by confessing his fear of losing you, he responded with a knee-jerk reaction of demanding that you not talk to your friend again.  

To resolve this dilemma, try communicating with your boyfriend about the incident.  Calmly approach the subject by asking him what he was thinking or feeling that led him to react the way he did.  Stand firm in your right to maintain friendly relationships with other individuals (male or female) and make it clear that his attempt to control these relationships is unacceptable.  Also, acknowledge that your boyfriend may need reassurance about your feelings and commitment toward him.  This conversation will likely bring up issues of trust, fidelity and boundaries in terms of what behavior is appropriate versus inappropriate.  Being able to discuss such issues and work together to find a mutually satisfying solution is the key to strengthening your relationship.