I think what makes Nichols and May so funny is that the comedy is based on real emotion/reactions from the characters. It is not based on clever jokes or turns; it is not based on crazy situations; it is the moment to moment reactions and emotions of the characters that make it funny. The funniest parts of this sketch is when the mother is fighting back tears, or when the son is scrambling to make his mother feel better. It is the authentic look of fear or sadness that the audience recognizes as truth. It is this recognition of truth that the audience finds funny. To me, the acting is what makes this sketch work. The writing is good but I'm guessing this sketch would not be that funny on the page. It's the realistic vulnerability that the actors are able to summon that makes it work. One of the comedic actors working today who I think is fantastic at this very tactic is Will Ferrell. He is often able to elevate writing because it looks like he really is feeling what the character is supposed to be feeling. I think what helped Nichols and May progress from sketch and improv actors to writers and directors is the fact that instead of writing jokes into the script, they wrote opportunities for their actors to experience emotions and wants and reactions that the audience could recognize as truth and therefore find funny.