How do I get my son to stop swearing at me?

Maybe offer him something else to say?

Our son briefly picked up swearing from somewhere, but because neither I or my husband swear they changed the words pretty quickly.

My favourite swear word is 'poppycock' (which is actually an old fashioned lolly!), and it's actually really cute to hear them use this word with such violent disgust!

Well, this can certainly happen with school going kids. They pick up new words and often get used to saying them without knowing their meaning. I think if the word is not very offensive then you may just ignore it when he uses it, eventually he will stop using it. However, sometimes it is very necessary to explain to the kid that using a word is not good and should never be done. I think once the kid understands that such words should not be uttered, he will stop using it. You need to tackle the issue very calmly and patiently.