Swearing

I have high functioning son who's 6. 

He's usually wellbehaved and social ( maybe too social sometimes) good at some things bad at others.

I've been very proud of him for not swearing, but he started with saying What the hell for a few days alot of times ( never in anger it just pops out of his mouth on any occasion) . Liie any parents do they say the usual thing that it's a bad word and dont use it or other things I ignore it.

I dont swear in english or basically never. When I get mad I swear in my own language which he doesnt understand. 

One day we went the playground and when we were heading home and to the gate we ran and I came first to the gate, he didnt like that and hates not to win. So he got angry and swore using the F word ( which I really hate and NEVER use ) ...I got really disappointed and told him off to not use that word ever again.

He didnt keep that word away.  He saw an edited peppa pig video and heard the word wtf and I took the tablet away immediately. And I told him it's not good word.

So now for a week he's been saying it. Never in anger just spontaneously. Its always like

We dont say what the f*** mummy 

Does the teddy say What the f**** 

Does the little girl say What the F**** , no mummy shes a good girl she doesnt say what the f**** 

I cant get the bad word go away mummy it wont go away 

I dream about what the f**** 

I'm a good boy I dont say what the f***

And so on and so on. He even wakes me up in the morning with telling me that we dont say what the f****

I LITERALLY cant deal!! When I tell him to use other words he just says " we wont say what the f**** we have to say other words like what the fudge instead and be a good boy" and then he can be quiet for a few min or hours and then seem upset that the F word wont go away. 

He cries when I tell him to not say it. He says he's teaching me not to say it. For me it's like now it's any excuse just say that horrible word. 

I avoid crowded places.if I see other children I get so anxious that he will slip up. I even had to let him not to go behind a family that was walking out of the station because I knew he would say " the little girl dont say what the f***" 

When we went home on the train there was alot of people on it and when it was all quiet he just said to me We dont say what the f**** mummy after being quiet for a while( he speaks his mind alot)Of course people listened and I just wanted to die. Hes been so wellbehaved for all our journeys on the train for many years but now all he wants to say the F word whenever he does.

It's gotten so bad because he is literally obsessed now. I just dont know what to do but to make him stay grounded for the rest of his swearing days. I cant do that but come on. Please help me here. He goes to a special needs school so I hope the teachers will tell him not to say that word.

I'm the end of my tether please give me some advice.