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The Bayfield High School pupil attended his first international champs, which are held every two years, and came away with a bronze, silver and gold medal.
He competed against 165 swimmers from 20 countries, including Sweden, Canada and Australia.
"It was fantastic. I didn't expect to win," he said.
Carlos has been swimming for nine years and despite butterfly being his favourite stroke, he won the medals in the 50m freestyle, 50m breaststroke, and 100m medley in the under-19 section.
He plans to attend the next championships in Italy in 2012 and has an ultimate goal of competing in the Special Olympics.
The championships were established, along with the Down Syndrome International Swimming Organisation because Down Syndrome swimmers were being outclassed in competitions by others who did not have the same physical disabilities, the organisation's website says.
Some countries have established Down Syndrome national swimming organisations to further the opportunities of their swimmers.
As New Zealand does not have such an organisation, Carlos was invited to attend the Australian national championships next year.
His father Sergio hoped a similar organisation could be set up here to support people such as Carlos in achieving their goals.