I am glad your DS is ok with it all.

I think it's hard to know just what has happened but i tend to agree with the others about not bringing it up.
It could be a total mistake and the mum might realise once she doesn't get your RSVP OR he might have been told to pick 5 friends and on that particular day he didn't think of your DS and his mum might just be leaving it up to him.

My DD1 and her bestie from kinder have days where they love each other, hold hands, don't want to leave each others sides then other days they don't really see eye to eye and are happy playing with other kids, if she wasn't invited to K's party i would be a little upset but i would just have to realise kids will be kids and i can't MAKE them continue to be friends.. IYKWIM.