From a psychological perspective, numbing one's feelings is a know method of self-management. It is in the general category of "resistance", not to be confused with something negative. It basically is when feelings are too varied or intense, a person might automatically numb themsleves out so that they can stay functional.
Most of us do this on some level when we need to. It is common when we lose someone suddenly to death. Sometimes it takes months or years for someone to grieve because it takes that long for them to be able to sort through their feelings and become strong enough to deal with them.
I am not sure if this is actually what is happening in your son's case. With a recovering addict there may be other issues. For examples, some drugs impact neurotransmitters that affect mood and other brain functions. Each drug affects a different set of neurotransmitters and some drugs create more damage than others.
Whether or not normal emotions return depends on a lot of factors. What drugs, how long, what was the mental state of the person before drugs, etc. I have relatives whose issues with substance abuse were triggered by the onset of mental illness.
Additionally, someone who has struggled with addiction normally has a lot to process in the aftermath. Depending on thier makeup and how intense the emotions are that need to be processed, it may take some time.
However, from what I have read, barring any underlying mental issues, most addicts recover normal emotional functioning as they recover.
Congratulations on your son being one month sober... prayers for continued recovery.