If your son does have at least two impairments listed by the ADA from ADHD its in his best interest to inform his employer. It is going to save a lot of hassle in the long run with employment.
People get upset. Thank goodness your son is an introvert because that can cause problems when you have a jerk of a boss.
As long as your son is a proletariat and providing his capable services for compensation then to err is human. There is no excuse for not rational approaching a problem in a collective manner to discuss what may have happened and seek out remedies to prevent future occurrences.
Your son cried from the confrontation. That could be social anxiety, lack of ability to delineate thought under pressure, or stress. It happens to the best of us. Reinforce the positive is the best a mother can do. You got a job, your trying your best, you not stooping to his level of aggression shows a lot on character. On the other hand it could be from your sons subconscious that he doesn’t have it under control completely or feel he is master of his mental domain. Sometimes we forget we struggle or to give ourselves pats on the back for the positives until we hit the wall and its to late. This could be a good thing for him in a sense of refocusing on the important things within himself. 
Cryings good. Not letting it out is troublesome… Good job though. Supportive loving parents make all the difference.
REPORT ABUSE