What stood out for me was this
“I even told DS to stay away from DH when he was in a mood. I can’t believe I did that”
I can.
I grew up with a stepfather who had a terrible temper and moods (was very Berts abusive to DM, me and to a lesser extent his bio daughter with my mother).
I was told to “stay out of his way” when he was in a mood. Which was a lot. So I did. I read in my room. I went out to play alone. I hid from him basically for 5 years. I felt like a stranger in my own home and between the abuse and the “staying out of his way” life was pretty shit. I fantasised about being taken into care. Thank god I was a reader. It saved me.
You say your DS is in his room more and is being grumpy - are you surprised?!? You’ve TOLD him to stay out of his stepfathers way, which as a child, in relation to the “alpha male” of the house means “stay out of the family space”. It will have made your son feel like HE is the problem. It will also have made him feel like you’re not on his side, and ultimately like he has no one.
I know this because I was your son 20 years ago.
Let this guy go. If things are bad enough with his “moods” that you’re telling your son to stay out of his way, it’s bad. And it’s bad for your son.
You’re both better off without him.