My step son does this, he’s 5. According to science It provides some (temporary) relief from the intensity of the emotions they're experiencing. They'd rather hurt themselves than take these feelings out on others. They feel they need to punish themselves, because they've been “bad," or because they don't see themselves as “good” or “worthy.” The fact he doesn’t see anger a lot from you is a lovely thing so don’t be hard on yourself. It’s all about teaching and learning at these moments. I find a calm talking session about what we’re feeling and why we want to do that helps him not too. Usually a big hug holding his arms and to explain what’s happened. I always remember as a child being told you will understand when you’re older and it never sat right with me so I explain everything in as much detail as I can and that it’s not his fault and we do things to keep him safe x
At his age he is way too young to manipulate you. Kids at that age do not have a cognitive level to understand manipulation. I will say I agree with the above comment about self punishment. I myself am autistic and I do that unfortunately especially during autistic meltdowns. My mom used to think I was being manipulative but i wasn’t I was just hurting internally real bad.
Kids are smart. He learned that escalating gets him what he wants. I would ignore it if it was my kiddo.