In 2015–2016, Hiwatashi debuted on the JGP series, placing fifth in Colorado Springs, Colorado before winning the bronze medal in Zagreb, Croatia. He won the junior silver medal at the Midwestern Sectionals, finishing second to Alexei Krasnozhon, and went on to become the junior national champion, outscoring Kevin Shum by 14.78 points for gold at the 2016 U.S. Championships. Later that month, he was selected to replace the injured Nathan Chen at the 2016 World Junior Championships in Debrecen, Hungary.