Personally, I think it was a pretty poor job. Coming up with a reasonable scenario isn't that hard. I have one here:
Bron was chased by Sharptooth. He was able to get away, but hit his head on a rock and lost his memory (he probably wouldn't know the word "amnesia") and he forgot who he was for many years.
Eventually, he remembers and goes searching for Littlefoot and his wife. However, he finds that his wife has been killed by Sharptooth and that his son is missing. He would pick up longnecks, looking for Littlefoot.
Also, the chased by Sharptooth idea could give his wife and Grandpa and Grandma Longneck the impression that Sharptooth made a meal of him (which would explain why they didn't go looking for him either, something the film also doesn't explain with Bron's reason for being absent for so long.)
Also, if Littlefoot thought he lost Bron to Sharptooth, and then lost his mother to him, it would make him very determined to kill Sharptooth like he did. (But, on the flip side, he'd be, one would think, even more skeptical of Chomper for that very reason.)