That's not how pregnancy works. :>) To get pregnant, you need to have sexual intercourse. Someone masturbating in the home is not going to leave random sperm around that will find their way into your vagina. However, it is perfectly reasonable to speak to the boys father and set boundaries. The boy can masturbate but needs to do so in his room only and not 'openly' (whatever that may mean). That is a private activity and he can do it alone in his room or perhaps the bathroom. You will not get pregnant from it, that's not the concern. But his privacy and your ability to not see it would be great.
Again, no risk of pregnancy from this type of activity and am slightly surprised you don't know this. If you have general anxiety, that is a treatable condition. I would talk to your doctor about it to navigate irrational fears. Take care