I wouldn't mind if he talked to strangers online who have no way of knowing us, but yeah, telling his mom and brother would bother me too, unless I agreed to it or it crossed the line. Things that I feel cross the line of privacy are like abuse, gambling away our house and bank account, suicidal threats, and things along those lines. Since a father and mother can still be in a man's life, ask him what he thinks it means when the bible says, "A man shall leave his father and mother and be united to his wife."
To me it means that he lives in a separate house from his parents and is head of that household. An Example: The bible says that a Christian should not take another Christian to a worldly court, cause what would that say about God's people- that we need the advice and expertise of the world in spiritual matters? Christians are to solve matters within God's family, not turn to the world, but turn to the bible. And since the church is the bride of Christ, she shouldn't take their business to outsiders. So in the same way, a man should be the authority to solve matters in his own house, not take it to an authority and household that he has left.
Now there's a difference between seeking advise in order to make an educated decision, and gossiping. As far as advice goes, would he be using their advice to make a decision, or asking them to make a decision for him because he's not capable? Because that would be a boy in a man's body seeking the guidance of the authority he has left instead of being the authority in his own house, with the bible as his guide.
I don't know how private the topic is, but if it is very private I would be really upset, but if is like you made him scrambled eggs when he wanted fried, and asked his mom for advise, a good mom would say if you want something done right do it yourself. lol. But he should ask you if you mind if he discusses things, but I mean you can't completely not talk about what's happening in your life.