He may have something more wrong with him.
Other than that fact, he also may have an extreme coping mechanism.
Oftentimes my SD will get in trouble. I've posted about this. She will get in HUGE trouble, and then a few minutes later she's smiling and laughing. I don't understand this!
What we've discovered (and it took a while to discover this) is my SD is VERY GOOD at "forgetting" or not confronting what just happened. She won't "let" herself realize what she just did. She has an extreme coping mechanism. She puts up all sorts of mental barriers. She makes HUGE excuses in her mind where it's not her fault, or she ignores the situation entirely.
What I did to discover this, is she had gotten in trouble for breaking a vase. Now, it really wasn't a big deal to me because it was a cheap vase. However, she broke it, and didn't tell us. She picked up the pieces and took them outside to the garbage so we wouldn't find out. Like I wouldn't notice that the vase of flowers that was on the table wasn't missing! She didn't vacuum up the glass though, so my husband stepped on a piece of glass and cut his foot.
My husband was furious (which doesn't happen often) and he yelled at her. About 3 minutes later (and I'm not kidding) she was SINGING and skipping and dancing around the house, while my husband was still trying to get the glass out of his foot. No remorse. None.
So we sat her down and made her confront what happened. It took a lot for us to do this. We had to get through all the barriers she put up. First, she had convinced herself that she didn't even do it! Here we were yelling at her and she was sitting there, convincing herself that she didn't do it!
After we got her to admit that she did it, we had to peel away all the excuses she had made for herself. We did this by asking her over and over why it happened. When she gave us an excuse "It happened because the cat made me" we said "That's not the reason. Tell us why it happened."
Finally, the tears started. And the whole story came out. She admitted it happened because she was doing gymnastics in the house when she knew she wasn't supposed to, and she kicked the table and that knocked the vase over and it rolled off the table and broke. No excuses this time.
After the tears, we got a real apology. And she started acting truly remorseful.
For us, we have to help my SD really confront what happened in order to get the right reaction. She's very good at pushing the blame elsewhere, or even convincing herself she didn't do it! It's not that she doesn't have any remorse, she does. It's that she's very good at not accepting the blame so a feeling of remorse is not possible. FIRST you have to take responsibility. Then the feeling of remorse will occur naturally.
I would bet that your SS had convinced himself in his mind that he didn't do it. That's why he didn't tell you the truth. Telling the truth was admitting that he did it. Then, once the truth came out, he has excuses in his mind that make it "not his fault." The fact that he couldn't make himself stop. Of course he could!
He needs to truly confront what he did, and apology notes won't work. Punishments don't work because he turns himself into a martyr (he feels he's being wrongly punished, and while he may suffer through the punishment, he's still convinced himself it's not his fault.)
He needs to admit to wrongdoing, and he needs to admit it to himself.