2008-09: Westerholm made his debut with Malmo's U20 team in the SuperElit league as a 16-year-old and scored 7 goals with 5 assists and 6 PMs in 21 games. Playing for Malmo's U18 squad with his twin brother Ponthus he was the team's leading scorer with 20 goals and 24 assists in 23 games. He and Ponthus led Blekinge in scoring in Sweden's prestigious TV-Pucken tournament for high school-age players with 10 and 6 points respectively. Pathrik had a team-high 7 goals with 3 assists in four games.
2009-10: Westerholm was the leading scorer for Malmo's U20 team playing alongside his brother Ponthus and made his pro hockey debut, skating in one game for Malmo in AllSvenskan (Sweden's second league). He was scoreless in his only AllSvenskan game. In 39 games in the U20 SuperElit league he scored 22 goals with 24 assists and 18 PMs. Malmo finished second in the South Division and lost in the playoff quarterfinals.
2010-11: Westerholm split the season between Malmo's AllSvenskan and U20 squads. In 34 AllSvenskan games he scored 8 goals with 13 assists and was +18 with 8 PMs (Ponthus Westerholm played 24 games with the AllSvenskan team). The Westerholm brothers combined for 112 points in finishing 1-2 in scoring with the U20 team. Pathrik Westholm scored 32 goals with 25 assists and 8 PMs in 26 SuperElit U20 games. Ranked 37th amongst European skaters in Central Scouting's final rankings, Westerholm was selected by Vancouver in the sixth round (180th overall) of the 2011 NHL Draft.
2011-12: One of two overage, late round selections from Sweden taken by Vancouver in the 2011 NHL Draft, Westerholm once again skated alongside his twin brother Ponthus with Malmo in Sweden's second league (AllSvenskan). He scored 6 goals with 3 assists in 43 games and was minus-four with 4 penalty minutes. Malmo finished seventh in the 14-team league and was second in the four-team pre-qualifying tournament to reach the Kval series. In six post-season games Westerholm scored 1 goal and was plus-one with 2 penalty minutes. The Westerholms shared the scoring lead for Malmo's U20 team in the SuperElit playoffs with six points each. Pathrik Westerholm scored 2 goals with 4 assists in five games and was minus-two with 6 penalty minutes.