I had this yesterday but with an adult! She is a school mum and we invited her eldest DD to DS party (as a return invitation rather than DS really liking this child). She then asked about her youngest and I agreed she could come. I offered to pick her kids up as they were doing an activity 2 mins walk from my home, to save the drive and she declined.
Then she messaged saying her kids wanted her to come to the party, and she was happy to have some cake. I said it was a kids party, but I would save her some cake.
She then insisted for H&S reasons, she needed to be around for her youngest (we have a big paddling pool in the garden). I sent a photo of the pool, and said it was still a kids party, and if she decided against her youngest attending the party, I understood.
She then said she would hang around discreetly and I had to say parents are not invited, I was starting to feel uncomfortable about her insistence and to please respect our invitation.
So now she has deleted me from social media and isn't talking to me.
And she is forty something. I give up.
Your poor DS. It must really sting to not be invited.
I would message the mum and say sorry for the text, and explain that I was feeling emotional because DS was so upset. I would also offer to invite the boy over and mark his birthday separately.
And when the party is happening, I would take him out for a treat to keep his mind off it.