@Emily1988,
Having three children, several nieces/nephews & friend's children, I have seen and heard it all. My go to response is to calmly speak to the child and state to them firmly, "In our home we do not speak to each other using that kind of language." You can insert whatever the inappropriate behavior that has occurred. In our home we do not hit each other, bite, pester the dog, spit, break things, etc. etc. It's best not to make a big deal out of it. Just be firm and clear with your expectations about what's appropriate behavior in your home. Some parents do not have a problem with their children swearing and that's their prerogative in "their home." I would not bother the mum with this issue until she returns from her holiday. Should the negative behavior continue, reiterate your expectations and have the child engage in some other quiet activity away from your son. You may wish to give yourself a time out if you are having trouble controlling your emotions and appreciate the fact that your son is not behaving in that manner. Good Luck!