It is pretty good for a fifth grader! Don't think I am berating him here. I am just pointing out a couple of different things that he could do to make it stronger. I am not saying that he should rewrite the whole thing, but I think that there are a few things he could jazz up. Right now it is sounding a little like a condensed research paper and less like a poem.
Perhaps he can add a metaphor, simile, hyperbole, or some personification. Has he learned any of these literary trick yet?
Gold nuggets were rare.
This might be a good place to use personification. Maybe something like: Gold whispered their names
Old men looked for gold
This line makes it sound like only old men were involved. Maybe change it to something like: Old and young heard the call.
Lots of people moved to California
He mentions California later in the poem. Maybe develop the idea of leaving their lives behind more: Leaving behind their families and homes.
Dust from gold is called gold flakes
This was the part that sounded most like a report to me. A little too factual and not quite poetic enough. How about something like: Dreaming of golden riches.
Fleeing to California
Everybody wanted gold
Leave these just as they are.
Very few people found gold nuggets
Every few people found gold flakes
Switch these around. The second line is a little awkward. How about: Very few found gold flakes/ Even fewer claimed gold nuggets.
Rare finds were good to the people of the gold rush.
End with a punch! Rare finds were just that ... rare. The vast majority of people did not come out of the gold rush rich. Come up with a line that summarizes the experience. It could be something like: Ruin more common than riches.
Good luck to him!