Ah, this bit is shit.
When he's in a brighter mood, I'd have a sideways chat with him. So, do something together and bring it up lightly. I'd insist on, what we call in this house, a bare minimum level of respect. Set it out clearly. No matter how emotional or pissed off he is, he needs to treat you and his step dad with the level of respect demanded. For me, that was not telling us to shut up, or to slam doors so hard that they're at risk of damage, he's allowed to be annoyed, but no swearing or anything like that.
I also read, when mine was a teen a psychologist saying 'I'm fine' is the hardest thing to overcome when talking to teens. So I told my teen this. That I won't believe 'I'm fine' if his body language and other things he was doing was telling me otherwise. I was clear that he didn't have to tell me exactly what was wrong (although if he did it would help!) but admitting that he wasn't fine was a great first step.
I also discussed privacy. That as he grew older I understood the need for privacy, but that secrets were different. I could always deal with the truth no matter how awful it seemed to him.
I remember when mine was about 16 I was in bed, and he can and slammed on the bed next to me and buried his head in my side. I didn't say anything, just cuddled up to him. And he said... I'm not fine. Breakthrough! He was having a fall out with his girlfriend and they were on the verge of breaking up. We'd been though this a couple of times before and he just slammed around and was pretty hard to be around. Every time, I told him that I was there for him and he could talk to me about anything. He never did, although weeks later he'd mention it. I had to put 2 and 2 together. But persistence paid off!
I also discussed teenage changes with him, and showed him some brain scans of teenagers and stuff and showed him how some of what he was going through is recognised, it's developmental. And I think that helped too!
And I always told him I was on his team. I wasn't his enemy.
Mines nearly 20 now and we can have a good giggle about some of what went on.