This man appears to be dumbfounded.
If you say something so smart no one can believe it came out of your mouth and everyone is amazed, this is an example of when you dumbfound them.
Origin of dumbfounddumb + (con)found
transitive verbdumb·found·ed, dumb·found·ing, dumb·founds, also dum·found·ed or dum·found·ing or dum·founds
Origin of dumbfounddumb (con)found