When ds started school at five, he didn't know any of the kids in his class very well, said he liked them all and wanted to invite everyone apart from one boy who has some behaviour problems. He said he was frightened of him and there had been two issues before when I had to speak to the teacher about hitting. Still, I couldn't invite a whole class and leave one out. I put it to ds that if this boy wasn't asked, he would be better just inviting four or five boys, leave out the girls and the rest, and he needed to put himself in the boy's shoes, how would he like it?
He thought about it and said he wanted to invite him. I then worried inwardly that I was foisting a boy he was frightened of on his birthday party, but knew it would be fine as there were loads of adults around to oversea.
All that angst for nothing, the boys parents didn't bother to RSVP!
Learned a lesson there.
Op, it's not nice, especially as the boys do play well together. Sounds like the mother has an issue with you, unfairly. Perhaps someone has spread unfounded gossip? Whatever, it's unkind but it is life. Just a shame your ds has to experience it so early, and in such a public way! I would hurt too.
Make a lovely day for him and to hell with them.