While I certainly don't think his pension was 'earned', he entered into a contract with his employer, and his employer entered into a contract with him. If he did the work and put in the time that was specified in that contract, then his employer owes him the pension that was specified in that contract.
Taking that from him is, indeed, theft.
Going forward though, new hires need to be given much more believable benefits.
“Taking that from him is, indeed, theft.”
No, it won’t be theft, it will be bankruptcy.