The best thing I can suggest without knowing more about the situation is “Sub Drop”. It’s a phenomenon that occurs in both doms and subs. Bassicly its an overwhelming feeling of pain, fear or depression and sometimes it manifests as anger and depression.
Most subs don’t even realize it’s happening so they don’t tell their doms and then it just ends up turning into an “I didn’t like that” kind of thing. While it is COMPLETELY possible he did not like it; Sub Drop is a reasonable guess as well.
I think you honestly just need to talk to him and find out what went wrong. His body is not a sure sign of how well he enjoyed the attention either. Some people just can’t help what their body does even by the slightest touch. Sit down and talk to each other. Out of Dom/Sub mode. Just talk as lovers or play mates.
Discuss what happened and when it turned into something he didn’t like. Ask WHY he didn’t use the safe word and try and correct the behavior. A sub who does not use their safe word when needed should not be subbing. It puts both parties in danger either physically or emotionally. If he continues not using the safe word I suggest you stop playing with him for your own safety and his.
Also, discuss sub drop and see if that’s what he is experiencing. Sometimes subs just can’t put their finger on what it is. Also, I’m not sure what kind of after care you do but I think it’s VERY important. It helps your sub come down more slowly and helps them switch over easier. I have a routine for Kitty and it tends to work well. She hasn’t experienced severe Sub Drop, as far as I know.
When we end a scene I like to have some water and maybe a fresh snack on hand and her favorite fuzzy blanket. We snuggle in the blanket and eat and drink water and talk about the scene. It’s a lovely way to end things and I’d highly suggest it along with any medical care your sub may need (If you’re playing hard). It’s just that one must keep in mind that kink is very mental and emotional. We aren’t just stressing our bodies.
I would just say try and figure out the reason as to why he had this reaction but don’t make him feel like he did a bad thing. Maybe he got really wrapped up in the scene and forgot. Maybe use some kind of verbal cue to check on him? But as far as I can see you did everything in your power to make sure he was safe and he didn’t follow the rules. Keep your system just make sure your sub is on board with it and figure out why he froze up.
Hope this helped