Maybe there is a compromise somewhere in here.

Could he stay somewhere else when he comes back for a while and then get a covid test before coming home? CDC says a person should get tested 3-5 days after exposure.

If that’s not realistic then you should explain to him exactly how much this worries you. That you know others dropped out because of the same concern. And come up with an alternative to celebrate his friends bachelor party in a different environment. Maybe they can do something 1 on 1 or with a very small group of vaccinated people in a less risky setting?

If this isn’t something he wants to compromise on maybe you need to stay somewhere else for a bit when he comes home to give yourself peace of mind.

The reality is you’ll both probably be fine. But there is always a possibility that won’t be the case. We all take risks at whatever level we feel is comfortable. It’s okay to not be comfortable with this.