Ok he's acting on assumptions.. that it's ok because... blah blah blah. you can remedy this. Instead of just kind of saying "well I dont' like people to know my private stuff" that's not enough. Work now to get some boundaries set, some rules of behavior that are acceptable. This is not a negative thing, it's a good thing. Tell him outright, that you do NOT want him telling anyone about such and such (be specific as to where your lines are drawn) unless he tells or asks you if it's ok first. Set it up so that he knows your rules. nothing wrong in setting those lines up. He will better succeed as your boyfriend that can give you what you need if you let him know what the rules are. Don't be vague. Don't just say I hate it when... "_______" that does nothing. you can tell him that you don't like it when he does this thing he does but you have to go further and show him what is ok, what the limits are. AT that point if he does something outside of those lines, he has to be a dimwit to expect that you'd be ok because you've told him you're not.

you say he should "know better" but I'm going to guess that you're assuming that he should, but like most people in relationships you assume based on the length of the relationship he should "know you" better. Not if the rules have never clearly been defined by you. no matter how long you know someone you can't know those inner values and boundaries of the other person unless they've been defined.