Pretty normal for lots of teenagers to do that- however, open and honest people can still have harshly strict views on some topics, and if the teenager doesn't know how to broach a subject to see your view they may become distant to "protect" themselves instead. I knew I was gay when I was 9 or 10, and in that moment, I felt completely alone. I had no idea what my family's take on it would be, so instead I put up a front that revealed "enough" (usually plausible sounding lies) to minimize interactions; I saw my family as a stressful prison, and my friends as my "real life" because I could be honest with them. When I put it into words like this, it is hard to see how it is different from a typical teenager, but I could see how extreme the distance between me and my parents were every time I saw my friends interact with their parents. They had their lies, but didn't come close to the level mine were on- and all for no reason.
My parents were completely fine with me being gay. I spent 8 years agonizing over every detail of my presentation, of my speech, of what I told my parents about where I was, it got to the point where it would be hard to breath when I was talking to them and had to be extremely mindful of keeping my breath seemingly under control to not set off any alarms. After hearing so many stories of gay children being abandoned by their parents, I became paranoid over how conditional a parent's love can be. I didn't see them as the people who always would love me- I saw them as the people who could stop loving me at any time.
My parents are liberal democrats who have never displayed any prejudice-ever. But since they never explicitly mentioned being cool with gay people, I felt the need to preemptively defend myself so I wouldn't end up being another sad story on the news.
That being said, there could be any number of things bothering him. When you are a teenager, you have zero power in relationship to your parents-the onlynthing you can control is the flow of information. Teenagers know the life you guys have, the status quo, probably won't change significantly if they share "risky" information and it is received well. But their life can get a LOT worse if they tell you something you don't like, so the obvious choice is to just distance yourself from family for safety reasons, tl guard the identity or life they have constructed. If there is anything you want them to talk to you about, you would have to lead the vanguard and show talking to you about it doesn't pose a risk, about that topic specifically. Being generally open doesn't encourage it because plenty of open people have their exceptions for what they are relaxed about.
Anyways, I don't know if that was helpful or relevant, just kinda reminded me of me when I was younger.