Swearing

I suspect he doesn't actually want to say the word but, as you note, he's "obsessed" by it and it "won't go away".  He's not generally using it as an expression of anger or frustration, or to fit in, as many older children do.  Has he been diagnosed with - or assessed for - other conditions like Obsessive Compulsive Disorder or Tourette Syndrome?  The latter is often particularly associated with involuntary swearing and repeating words.  But of course repetition of various types is very common with autism.

I think it's easy for anyone - even without these conditions - to become obsessed by trying to avoid any specific type of behaviour or intrusive thoughts (and speech must on some level be motivated by thought).  The more we are told - or tell ourselves - NOT to do something, the more it reinforces the original thought.   As a young boy, I often used to wet the bed and my mother instructed me to keep saying aloud, "I will NOT wet the bed," as I put my head on the pillow.  It didn't work and I only became more terrified of and obsessed with wetting it.   

It could be something as simple as the attraction of "forbidden fruit" which isn't even specific to autism; the more we are told not to do something, the more interesting or appealing it becomes.  It seems you've already tried substituting another word like "fudge" but maybe that line is worth exploring further and rewarding him in some way when he says the substitute. 

I can say hes got a bit of ocd with closing all the doors in our home before we go outsude for example. If he misses that with being occupied with other things then he starts crying.

When we are downstairs in the communal hallway he has started to touch everything in a rush and going on step at the stairs before we head out. I can break those habits sometimes with letting him carry something.

Hes had his ocd since starting school last year, I think he gets very anxious before going to school so the ocd gets triggered before hand.

Should I just ignore when he says the bad word? 

I don't know but ignoring it might be worth trying if all else has failed and particularly if you sense he's getting a buzz from your or other people's reactions?

Perhaps to mitigate the behaviour, you could also try cultivating his sense of the difference between the public and the private.  I don't know at what age children typically learn this - I recall I was still doing slightly "inappropriate" things at school when I was ten, perhaps due to my then undiagnosed autism.  But by our teenage years most of us have gradually realised we can say and do things in private that we can't in public and for autistic people especially I think the "safety valve" of being able to let off steam when alone or at least at home is important.

Thinking further about this, I think it's important to recognise that, from what you write, most  times (except the first) he is not actually "swearing" but using the word in invisible quotation marks to pursue two goals that are particularly important to many autistic children:

i)  gathering data on the subject (does teddy say it? does the little girl say it?) as if it's become his special interest.

ii) he's told you that "he's teaching" you not to say it, repeating back your own instruction and effectively becoming what's sometimes called a "little professor" on his admittedly very narrow specialist subject.

So, in a sense, he's adopted a scholastic and educative relationship to the word.  This may not alleviate other people's discomfort, but it's as great as the difference between shouting the word at someone as abuse and analysing it in a discussion of linguistics.

By the age of six, I'd developed special interests in geology and archaeology and delivered "lectures" on them to anyone who'd listen (or wouldn't).  Not because I was a genius - the audiotapes recorded by my parents prove it was mostly garbled regurgitation of stuff I'd heard or read - but because I had this overwhelming need to learn and educate others about my special interests.  Perhaps he can be steered towards - or further encouraged in - other subjects that will distract him from this one and are considered more socially acceptable for his age?  But, who knows, maybe one day he'll be a professor of linguistics...

Thinking further about this, I think it's important to recognise that, from what you write, most  times (except the first) he is not actually "swearing" but using the word in invisible quotation marks to pursue two goals that are particularly important to many autistic children:

i)  gathering data on the subject (does teddy say it? does the little girl say it?) as if it's become his special interest.

ii) he's told you that "he's teaching" you not to say it, repeating back your own instruction and effectively becoming what's sometimes called a "little professor" on his admittedly very narrow specialist subject.

So, in a sense, he's adopted a scholastic and educative relationship to the word.  This may not alleviate other people's discomfort, but it's as great as the difference between shouting the word at someone as abuse and analysing it in a discussion of linguistics.

By the age of six, I'd developed special interests in geology and archaeology and delivered "lectures" on them to anyone who'd listen (or wouldn't).  Not because I was a genius - the audiotapes recorded by my parents prove it was mostly garbled regurgitation of stuff I'd heard or read - but because I had this overwhelming need to learn and educate others about my special interests.  Perhaps he can be steered towards - or further encouraged in - other subjects that will distract him from this one and are considered more socially acceptable for his age?  But, who knows, maybe one day he'll be a professor of linguistics...