A junior product of two clubs: Gries Oberhoffen and Nancy, Westermann attended INSEP before turning professional with ASVEL Villeurbanne in 2010. After two seasons in Lyon, he signed for Partizan Belgrade and helped the team to Serbian and Adriatic League titles in 2013 and 2014, alongside compatriot Joffrey Lauvergne. After recovering from an ACL injury, he signed for FC Barcelona in 2014 but was loaned for the season to Limoges. Called up as a late replacement for EuroBasket 2015, he debuted against Great Britain in a friendly in Orleans in 2012 and has made a total of five appearances. However he was part of the French side which won silver at the 2012 U20 European Championship, where he was also named tournament MVP. Additionally he also claimed bronze in the same tournament in 2011, as well as silver at the U18 European Championship in 2009.