We were in a somewhat similar situation, I can't say we have resolved it but here are some thoughts:
- speak to the teacher to understand how this plays out at school; in some schools the teachers encourage them to play with more kids than just the best friend, not sure if this is the case in your school;
- come down hard on it when it happens with you/at home (i.e. no TV for one week), so they understand it is not acceptable
- try and expose him to kids who are well behaved, they mimic a lot at this age; for example cousins, or other school children that you think are well behaved;
- yes, invite the other child over and set limits for the other child too; but I would not invite him over a lot, you want your son to have other friends too (without talking badly of this boy in front of your son)

I am interested in other suggestions too!

good luck!

I feel your pain - been there and done that!

From my side this is what we learnt...

1) it's often not only one boys fault. I assumed that my boy was being easily led and it wasn't until I saw irrefutable evidence that mine son was, if not the ringleader, then at the minimum quite happy to tag along that I realised there ere quite a few making it happen

2) agree about different parenting styles - nightmare!

3) I found that the best way to deal with it was to welcome the other child over and do the things they loved (activities/meals etc) that meant that I had some weight when I could say to him "We don't always behave like this, do we?" To be clear I was parenting the other child, just making sure he knew I didn't approve then I would turn off the playdate tap for a week or so and then welcome him back with open arms. Trust me, if he gets his favourite food/TV show/activity at your house and he's bright he'll quickly realise that actions have consequences

4) it is very easy to get into a stress about this and what I found worked was to tell my child to behave, to be kind but have zero tolerance for bad behaviour. So if they start to mimic what they've seen you come down on it hard, they often come back with a "but XXXX does it all the time!" and my response is "and you're not X, so we don't do it". Again kids are pretty good at understanding that not everyone behaves the same. They try to take advantage of bad behaviour as something they can copy and blame but they're not stupid...

5) it will blow over. It stressful and awful but you'll be laughing in a few years about this. Just ensure standards at home are kept up and all will be well.

Now when they come back from a friends "dirty" house with nits every playdate - THATS a dilemna!!!

I do need some advice. My eldest started primary school last year and bonded with a really lovely boy. A few months into the school year a third little boy joined their group and ever since that, I have noticed changes in my son that do worry me - he's using rude words, he's never used before and has become quite aggressive towards his little brother. I am also worried that the new friend is trying to alienate the other friend. We had a play date at home with all three of them and it was quite obvious. The new friend is a very bright child but very challenging behaviourally. He has no regard to other people and is rude to adults and children, but my son seems to be completely in awe with him. From a brief encounter with the dad, it looks like we're not on the same wavelength, when it comes to parenting.
I really don't know what to do. I have been told that the new friend, has no other friends and really don't want to be alienating him, but at the same time I am worried that his influence on my little boy is not good and the other friend is a bit left out of what used to be a beautiful friendship.