@anguishedmum that does sound like a good first step I am glad to hear he understands he was wrong. But respectfully, I would highly recommend you read “Why does he do that? By Lundy Bancroft” very closely, as soon as possible. He also appears on podcasts and YouTube videos so that might be another way of hearing his take on these matters. I know it’s been recommended on here already, and that someone shared the PDF, but one thing in your update is well addressed in that book. By reading it you will be well prepared for a real conversation with him, beyond just assurances from him.
It’s the “totally out of control for no reason” part. He might really think this, but in his book Bancroft does a good job at outlining why the “going out of control” excuse is probably not true. Abusing someone is a way of getting control over them not losing control, there is something in this process for him that he finds rewarding.
The abuse pathway includes the happy phase when all is well, typically followed by a build up of tension (abuser might get snappy, impatient, quiet, or just withdrawn for a few days or longer), followed by either the threat of emotional abuse/violence and/or actual abuse or violence. This often followed by heartfelt (seeming) remorse and assurances. This is followed by a happy period where you may even be closer than before. The happy period can last hours, days or even months before the cycle happens again.
Bancroft attributes this to a couple of things. First, there’s usually an underlying attitude, typically towards women, that it is their role to serve men, to be kind and comforting and not challenge them. This attitude means their pride becomes more wounded if they are challenged by a woman rather than a man.They may feel safer intimidating or hurting a woman, and they may feel more justified because women are supposed to facilitate them (in there kind). This must be challenged or it could poison his attitude for the rest of his life.
Second, there is something to be gained by him from the abuse, for example even if he acknowledges he’s wrong he may have done a good job at deterring you from saying or doing whatever upset him in future, he has taught you what subjects requires extra sensitivity in future so his ego isn’t hurt- this is a win for him.
Another benefit could be that because his behaviour was so awful he knows you will go out of your way to put it behind you both and will make an extra effort to be kind, supportive and positive, you may find yourself doing extra things for him because you are so relieved the badness is over. The big problem is, you may also find this relief quite rewarding which might make you extra motivated try to preempt what will upset him in future, meaning your “good behaviour” may be even better for him than before and last longer. If it stopped there, it might be okay, but the demand for “kindness” is often bottomless, and the triggers can become smaller and smaller.
This is a big problem, because he is still so young if he doesn’t unlearn it now there’s a chance he will bring this approach into other relationships, and will destroy any chance of true closeness.
The book has good advice on what strategies you can use to avoid this dynamic.