Willie Bessi Devis was playing with friends near a waterfall in Barana when he came across the tag of Private First Class Dale William Ross, who served during the Second World War.
Private Ross was reported killed in action in the Guadalcanal campaign - one of more than 7,000 Allied losses - with Willie's discovery leading to his remains being found and returned to Oregon for a burial 77 years on.
Charles shook the boy's hand and exchanged words with the youngster following a ceremony on Commonwealth Street, where he laid a wreath in remembrance of the scouts and coastwatchers who served during the conflict.
Willie, who attended the private's funeral in Ashland, Oregon, in September, said the waterfall was one of his favourite places to play.
"My friend was running and I saw it in the ground. I took it to show my father, who said it was a dogtag," he added.
Willie and his father Henry returned to the jungle trail and found a second tag and bones and kept them safe, while another local Lloyd Onorio used social media to find relatives of the private.
Members of the Ross family came to the Solomon Islands and DNA testing by the US Army later found the remains on Guadalcanal to be those of the missing private.
The Old Guard Riders - a veterans assistance organisation - gave Willie a leather waistcoast and his own dogtags, while Henry said he was "very proud" of his son.
Hundreds of people turned out for a glimpse of Charles during his visit to the memorial on Point Cruz, built in 2011 in recognition of those who gathered information on Japan's movements as they fought against the US, United Kingdom, Australia and New Zealand.
Earlier in the day, Charles was greeted by cheers as he went to church on the last leg of this royal tour.
He attended a service at St Barnabas Anglican Cathedral in the capital of the Solomon Islands, and lit a candle for victims of gender-based violence.
Outside, as temperatures passed 30C, Charles spent around 20 minutes meeting well-wishers.
He went on to Government House to meet with the Governor General David Vunagi, a retired Anglican Bishop, and Prime Minister Manasseh Sogavare.
The prince, on his first visit to the Islands, also carried out investitures and was entertained by Fula'aro, a panpipe group who served up renditions of UB40 hits Red Red Wine and Kingston Town among other tunes.
He met members of the group and joked "It looks like you need a drink".
The prince's tour comes to an end on Monday, while the Duchess of Cornwall, who accompanied Charles throughout New Zealand, returned home earlier as planned.
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