Hi Trista,
Are you able to still contact the work colleague? If you know his registered post or last known address, write a formal letter to the colleague asking him to remove his car from your yard within 30 days (or longer if you're nice people) otherwise to contact you for further arrangements. In the letter write that if you do not hear from him within 30 days of sending the letter, he will have forfeited all rights to the car and you will remove and dispose of the car, at your sole discretion, from your yard and charge any costs associated with its removal or disposal to him. If you also know his email address, it is best to also send a copy of the letter to his email. Best to attempt two contact methods.
At the end of 30 days, if you have not heard back from him, you are free to dispose of the car.