He has previously played for the Castleford Tigers, Hull FC, the Warrington Wolves and Wakefield Trinity in the Super League, and for the Toronto Wolfpack in the RFL Championship.

's marquee signing for Super League XVI, with the Black and Whites paying a club record fee to secure the services of the former Castleford Tigers man.

The decision to allow Westerman to leave for free raised eyebrows given he was less than 6 months into a three-year contract and the Wolfpack had paid a transfer fee of £130,000 to secure his services.

[15] In the local derby in June 2019 against Hull Kingston Rovers, he dislocated his knee in a tackle only to slap it back in place and continue playing.

"[28] Ahead of the 2024 season, Westerman was appointed Castleford co-captain alongside Paul McShane by new head coach Craig Lingard.

[32] In December 2024, new head coach Danny McGuire named Westerman in Castleford's senior leadership group for the 2025 season, with Sam Wood succeeding both him and the departing McShane as captain.

[35][36] Westerman made a return to the England team on 25 June 2021, playing against the Combined Nations All Stars at the Halliwell Jones Stadium.

[37] In February 2023, Castleford Tigers fined Westerman after an online video was released showing him performing a sex act in public on a woman, who was not his wife.

The club also ordered Westerman to help educate young people on the effects of alcohol and "dangers of social media when in the public eye".