Our previous developer set his name in the license of our open source project.
Thats how license header looks:
Copyright (c) 2018 (his name), Stichting (our company name) <(his email)>
Can we change that to our company name?
If he was an employee of the company and used company property/time to write the software then the copyright would belong to the company.
Unless their employment contract gave him ownership rights of the software they produced. You can modify it however you want.
Since the code is open source, you may need to get permission from outside contributors to change the license if there are any.