Ahh loads of replies overnight I’ll try to get through them. A mixed bag I guess. I’ll address some of the posts directly. I appreciate all of the input however brutal.
This really isn’t about my son not being friends with the children. It’s the fact that he has been singled out. As he’s been to numerous parties and never ‘ruined’ any, I just think it’s very unfair. I always stay with him unless they explicitly tell me to go but at this age that’s not many parties for any of the kids. Other than knowing he has adhd which they wouldn’t know unless I had told them, they probably wouldn’t even know much of what goes on at school. I know this parent and she is meticulously organised so I can also be sure she would have chased up especially given we only spoke a few days ago. When we pick up from school one of the boys who had this joint party in particular runs around with my son a lot. They ply together a lot and this has been mentioned by the teacher. So it would be very odd if my son suddenly went from being a very good friend of this boy to hurting him/being nasty. My son has many faults but he isn’t a mean or violent person as he has been on the receiving end of it. If any of the children had experienced this I’d like the parent to bring it to me if it seemed I was unaware. I’d much rather they tell me and explain that than deliberately exclude him.
To HotChoc. I’m very aware of the behaviour since I’ve been struggling with it a lot longer than they have. As I mentioned above - my sons behaviour doesn’t involve violence towards any other children and as far as I’m aware he’s never shouted in any of the other children’s faces. We are trying to get funding for a 1-1 so that they can sit with him and its hopefully then not as big a disruption but it looks unlikely. This is a big issue but as they won’t provide it then what am I expected to do? I’m sure as a teacher you know how dire the funding for things like this is.
On the back of that, longlongsummer - I haven’t attacked the parent in any way whatsoever. I just feel sad for my son. As I’ve already mentioned we also did have a party this year and everyone was invited. I’m not sure why you think I haven’t acknowledged the behaviour or impact. I have explained how he acts in class and said I’m aware it’s disruptive and that he is perceived as naughty by other children. We have a lot in place and are pushing for more but I’m not there, obviously.
To another pp that asked, we are looking into medication. Doctors are extremely reluctant due to his age which I suppose is understandable but I’ve also heard good things about it so would be willing to try it if they would
My son is nothing like some of the violent children you are describing. It certainly is disablist to assume all children with adhd are the same.
RicStar I thought that exact thing regarding attendance. But it really was all of them. Perhaps it was arranged a long time ago so that everyone could go? It was also a Sunday so perhaps no clubs? I’m not sure. Not even sure if handouts were given out at school given we didn’t receive one.
To those mentioning hobbies: He’s already in a martial arts club, obviously it’s not full on yet but it has helped him a bit. I am a full time student and I commute an hour each way so I have very little spare time. He goes to the before/after school club where he has made a few friends. He really enjoys it there. None of those make a difference to his energy though 
I’m going to do as posters suggest and find something special for us to do this week when I’m getting him from school. I’m not sure what yet but he will be happy. I’ve also written a little note in his book explaining the situation a bit. I don’t expect the teacher to get involved at all but if he’s affected today she should know. Couldn’t keep him off today I’m afraid as at uni.
I really don’t know how to broach asking the mums without being rude. I also don’t want to be that entitled parent. If it has happened how I suspect then what can they possibly say that will make me feel better rather than worse?
Hope I haven't missed anything important!