Hiccup can see that this conversation is turning sour quickly. In a panic, and unsure what ELSE to do, Hiccup suddenly jerks Gomamon right off his shoulders, and stuffs him down the front of his tunic. The shock ALONE is enough to silence the Digimon, causing him to go rigid.

Then Hiccup looks up at Once-ler, unable to say anything for a moment with his shirt and vest distended and full of Gomamon.

...

"I....I'm....so sorry."