My fiancé has been working at a place for over a year, it came to light they weren’t contributing to his pension. They’re now refusing to back pay, this is illegal?

We are in the UK, he works 12 hour hard grafting physical labour shifts. They confirmed to him they’ll be contributing to his pension going forward but not back paying. They also didn’t “notice” they weren’t paying his pension, it was an employee that bought it to their attention. He won’t say anything and he doesn’t want me to. How can I secure his pension is back paid?