Dumb doesn't mean just one thing.
From Mirriam's Dictionary:
2a : lacking the human power of speech dumb animals
2b : of a person, now often offensive : lacking the ability to speak
3 : temporarily unable to speak (as from shock or astonishment) struck dumb with fear
4 : not expressed in uttered words dumb grief
3 : temporarily unable to speak (as from shock or astonishment) struck dumb with fear seems to work very well and serves as a proof of sorts that Jeb had "that" look while at his father's service. Was his use of dumb when talking about Jeb a wayback machine prediction?