Souwer first went to Japan in 2002 to compete in the S-Cup 2002 shoot boxing competition where he became the winner after defeating Chinese sanshou title challenger Zheng Yuhao.

[6] He won the Shoot Boxing World Tournament 2012 in Tokyo, Japan on 17 November 2012[7] After beating Kem Sitsongpeenong by majority decision (29-29, 29-28, 30-28) at the quarter-final stage and knocking out Hiroki Shishido in the semis, he defeated stablemate and student Henri van Opstal by unanimous decision (30-29, 30-29, 30-28) in the final.

[11][12] He lost to Dzhabar Askerov by split decision in the main event of Yokkao Extreme 2013 in Milan, Italy on 26 January 2013.

[15] There was some controversy leading up to the event, with Souwer saying of the Pancrase representatives "in Holland, they are what we call B-level fighters" and Shoot Boxing founder and president Caesar Takeshi storming out of a pre-fight press conference, apparently so upset at Pancrase's disrespectful selection of opponents that he could no longer bear to be a part of the proceedings.

[24] However, he was unable to compete after suffering from a bout of appendicitis in the weeks prior to the event which forced him into hospital where they had to perform a routine but emergency surgical procedure to remove the troubled organ.

[27][28] Souwer made his professional boxing debut on 17 March 2014, winning by TKO after his opponent Paata Varduashvili retired in his corner at the end of the fourth round in Den Bosch, Netherlands.