I can't speak to the chemical responses, but I can tell you how it was for me and for others who've shared with me.
I was an emotional drunk/druggie, and when I first got clean, I had to fake an emotional response. Well, I didn't have to--but felt compelled to, as I was a good actress and wanted to fit in. It took a while before I could connect to anything other than the fact that I was clean in the present moment. I refused (or was unable) to think about my "wreckage" and any new emotionally provoking situation didn't have much of an affect. I can see now that there was a lot of ego present in my attitude. I was clean. What more do you want from me? That I do things that are unacceptable is of no consequence. Can't you see the impressive fact that I'm not using?
I was able to reconnect with emotions once I got into step work with my sponsor. With some humility, I could see that I wasn't god, that there were folks around me who were affected by my actions, and I set out to right (to the best of my ability) the wrongs of my past and try not to create new ones in my present. I'm on a pretty even keel today, though I do have deep and authentic feelings. I don't get overly anxious about the future (I said "overly!"), and I don't carry many regrets or resentments from the past. That's a product of step work. Emotionally, I can only live one day at a time. It's in that window that I deal with what's on my plate. I don't know if your son is in a 12 step program, but if he is, and depending upon where he is in his steps, he will eventually come to dealing with all of it--past, present and future--according to the morals that he's defined for himself.
Peace & Love,
Sugah