I'm glad you've decided to write him an e-mail apologizing, because I was quite angered by how you handled this whole situation. I don't see the problem in your boyfriend's behavior. But then again, it sounds so much like me that of course I would say that. But ASD or not, he was sick and feeling vulnerable.
The problems I see are the ones you didn't pose: 1) Expecting a sick person to open the door for you when you could have just brought keys, 2) Grilling someone who you clearly just stated is enduring what you judgmentally called "an emotional regression" (is it your place to judge that?), 3) You didn't consider what he needed at the time and bailed, 4) You seem to be judging his way of processing emotion by calling it emotionally constipated - which is amusing because its an example of being on the spectrum, so you indirectly just called a large portion of the people here emotionally constipated in the process of that statement. I don't know if you're aware of this, but it struck me that way.
Another note: It isn't "unhealthy" for someone on the spectrum to process emotion differently than you. I have a great way to describe my particular expression of handling emotion differently than NT's. NT's handle emotion in "real time," and I handle it in "post-processing." This isn't a choice; its a way of being. My emotion decides when it will operate on its own accord, and that is usually (though not always) later than in the moment, often days, weeks or even months or years later. Also, it does behave more like an on/off switch than a dimmer switch, meaning that when it does process, its an implosion of sorts. The "lack of real time" can also be an asset under crisis when most people are overly emotional in the moment.
Sorry if my response isn't what you're looking for. I'm a direct person. You came here looking for feedback. I've been on his end of this before, and I would not have put up with your behavior for long.
I'm happy you've come around to seeing his side a bit more. Also, the abusive situation from your previous experiences sound like a very temperamentally different person than your HFA boyfriend, so keep that in mind next time you're tempted to over-react.
I have no issues with how he processes emotion on a surface level... the problem is, he mutes me for having them and gets annoyed when I cry, for example... he has maladaptive coping mechanism (i.e., excessive drug use) and that makes me wonder if he's processing his own emotions in a way that's really serving him.