Your Ex's employer should have insurance for the work truck. Even if he had his own auto insurance, would not apply, since he does not own the truck, his employer does.
If you have your own collision coverage on your own auto, file a claim with them, then your insurance will go after the works' insurance to repay back for the damages that they pay. But, if only liability, then you will need to call the employer and ask for their insurance to file a claim. Or, my guess is that the registration is in the truck, which would show the name of the insurance company and policy to file a claim.
With a rear end, clear fault on your ex, thus insurance pays. But, once they find out, then they will see front end damages on the truck. Will he lose his job for having an accident, that is only what his employer chooses to do, but, since using his truck for personal use, then he could have an accident anytime, while not working, thus once again if at fault, then insurance will pay.
retired auto adjuster