2010-11: Wilhelm Westlund skated for the Farjestad U18 team and was named best defenseman while playing for Stockholm 1 in the TV-Pucken tournament for high school players in Sweden. He scored 2 goals with 11 assists and was +25 with 18 penalty minutes in 35 games for Farjestad U18. Westlund had 1 goal and 1 assist in 8 TV-Pucken games for Stockholm 1.
2011-12: Westlund moved up to Farjestad's U20 team as a 16-year-old and represented Sweden in the 2011 U17 World Hockey Challenge. He scored 3 goals with 4 assists and was +3 with 12 penalty minutes in 33 regular season games and was -5 with no points or penalty minutes in six playoff games. He also skated for Farjestad's U18 team, scoring 5 goals with 5 assists in 15 games, including two playoff contests, and was +14 with 10 penalty minutes. In six games for Sweden at the WHC he scored 1 goal with 1 assist and had 6 penalty minutes.
2012-13: Westlund made his pro debut in Sweden's Elitserien, appearing in 26 games for Farjestad, and skated with the club's U20 team as well as representing Sweden in the 2013 U18 World Junior Championship. He scored 1 goal and averaged 6:38 minutes of ice time; finishing -1 for the second place club. Farjestad reached the playoff semifinals but Westlund did not appear in any games. He scored 3 goals with 12 assists in 33 regular season games and was +10 with 76 penalty minutes for Farjestad U20 and was -1 with 3 assists in five playoff games. In four games at the U18 WJC he was +1 with 1 assist as Sweden finished fifth in Sochi. Westlund was ranked 15th amongst international skaters in Central Scouting's final rankings prior to the 2013 NHL Draft.