Alright, this isn't the normal topic in these particular forums, but I'll try to help.
Jen, sometimes when a guy likes a girl and gets worried about her he stops thinking clearly. This is especially common in guys who do not handle emergencies well(generally speaking of when they are unprepared) or otherwise panic easily. The proper way to do what he intended to do would be to create a new account just for him and create a thread that did not use your name. So, to simplify the many points I could make, I'm going to narrow this down to two potential ways that you could take this and the choice is your's. You can see this positively or negatively.
Positive: He was trying to help. Even if he went about it in a way that was less then the best option, he had your best interest at heart. That is much better then a guy who just says "get over it" and does nothing at all to help you. Even if this ends in failure, it is a sign that he does care about you. My advice, from this point of view, would be to forgive him, explain to him how this makes you feel and move on. It's hard, nowadays, to find people who have enough heart to care for someone else.
Negative: This could be taken as a betrayal. Without your permission he used your account, used your name, and shared information that you may have considered private information. There is still a bright side in saying that at least he did not try to claim that he was really you when he did this, he did explain that he is your boyfriend, but that does not change the fact that he did what he did not have the right to do.
Honestly I would consider this a very minor thing and thus would look at the positive side, but as mentioned I'll leave that up to you. I hope this helps.