That remark is insensative and your boss didn't overreact. Unfortunately people often say things that hurt so much, because they don't know what to say. Rainman may be the closest they can relate to autism. The reality is as parent of autistic children, our heart is broken, there is nothing fun or to be joked about that. We can laugh about my children's oddity with another parent of autistic children, but not others.
I would say your former boss is disoriented with his son's diagnosis. He doesn't want to go home because seeing his son is too painful. He gets nothing done because he can't put his son's autism aside. My hu*****and used to stay home as much as he can. Now he is at work 10 hours a day. After a while your mental health and physical health go downhill. It is a hell that no one wants and unfortunately we are chosen to live in, just because I, like everyone else, want a baby of my own.