my personal thoughts:
(if Shorty somehow wasn't adopted by pair of dinosaurs, of course...)
I tried to read Shorty's mind, try to put mysself on his place.
His personality won't let him leave Bron like that. Even if Bron heavily ordered him to leave with herd to safe place - after some time, when Bron is nowhere to be found and his fate is unknown...
There is still huge way to the valley, so I think, after earthshake and eruption is over and herd is going to go to the Valley, Shorty wouldn't go with them and go to rescue his adopted dad by himself, without going to the Valley. He won't waste time, he wouldn't go too far from place where Bron got missing, as Littlefoot didn't want to waste a time, disagreeing with Cera's way and wanting Gang to go instead sleep.
And since nobody from Bron's herd tried to look for Bron, I don't think somebody would leave the herd and look for Shorty.