In your situation, yes, it sounds like you had a good relationship prior to this and he got in way over his head at a young age when he was still developing and figuring out life and he made a bad decision, which he now regrets, and he's doing what he can to make amends. Yes, I would be proud of my child for admitting their mistake and trying to make it right.
I am NC with my mum, and while I don't have any interest in a relationship with her in the future, I do wonder really what she says to other people about me and our relationship. I do agree with you though that it's the choice of the people involved that the relationship didn't work out. It is because of your ds, not his ex, but it may be hard to get his head around that still.
I am NC with my mum in part because of her relationship with her partner (he is controlling and has isolated her, but he also poses a risk to my children). When I said we could no longer have a relationship with him because I needed to keep my dc safe, it set her into attack mode - because he is the most important thing in her world, and any threat to him is a threat to her and her sense of self and stability. But controlling partner or not, the reason we are NC is because of her and her behaviour, not her partners.
All that said though, I do have to say I think it's unlikely in a good, happy, healthy relationship for a child to go NC out of the blue, unless they've been sucked into a cult or fallen quickly into addiction or having a breakdown or something. I often think about what it would be like for my dc to choose one day to be NC with me, and I just can't ever imagine it happening. Because I know how difficult it is and the lengths I went to to save my relationship with my mum. I couldn't imagine my dc raising a concern with me or asking for help or reaching out the way I did and me not wanting to do everything I could do to heal that relationship.
It may be that you need to look at yourself and your family too. It was clearly running from something and yes, he accepts it wasn't the right thing to do now, at the time, it clearly felt like the answer. I think it's worth trying to understand that more, and that includes what you could have done differently. I'm a big believer in talking these things through. And also family is so important - and short of situations (like mine) where safety necessitates a relationship ending - I think most family pain and ruptures can be healed if everyone is committed to working through them.