CandyCrush77, I'm sorry about the hard time you are getting from this thread, although some posts have good advice and give you an insight on other people's perspectives.
People are different, and react in different ways in social circumstances. I can totally see your point of view, I can put myself in your shoes and see why your are feeling the way you say, and why you have decided the things you have.
About a year ago I was in a very similar situation, a child that was very friendly to us, the mother was also nice and friendly, so I kind of expected that if he had a birthday party he would invite DS. About a month before his birthday (and party), one mum asked us if we were going to this child party as she was working that day and was wondering about whether to send her son or not. We said we hadn't heard of the party and she said okay. No other comment. I then assumed DS had not been invited. I was puzzled and confused just the same way you are feeling now. I wondered if the invitation got lost, why he had not been invited if they seemed to play at school every day, and the child was always friendly to us whenever we saw each other, and the mum as well!
After going through all the thoughts you have been having, I eventually took the decision to remain friendly as usual with the mother and child, as if nothing had happened. I put it in a balance, and decided they seemed genuinely friendly and I liked them, so I wanted to remain friendly. I had to accept that there was a reason I didn't know, and that it had nothing to do with us as otherwise they wouldn't continue to be so friendly to us.
Then, as it turn out, two weeks before the party the mother sent invitations to the school, and DS got one in his school bag. So it seemed he had been in the party list after all, we just didn't know because the invitations were not out yet. And the reason the other mum knew about it a month before was because they were close friends and the mother probably mentioned she was thinking about doing a party on that weekend.
So even though the outcome for me was different, at the beginning I felt like you. As other people have mentioned, don't rule out it could be a misunderstanding. I remember reading a thread on mumsnet where someone said that a mother got upset with her after their child's party because her son had not been invited. And this person said that was not true, that she had invited her and she was the one that never replied. But then when she went to check her email account, she couldn't find it in her sent tray and realised that somehow must have got distracted when sending the emails and thought she had invited this child when in reality she hadn't. I know it sounds very unlikely but it happened to at least one person, so it still could be a misunderstanding.
If your son finds out somehow, either he won't see anything wrong with that (highly unlikely but children can surprise us), and if he does, all you can do is be honest with him if he asks why he wasn't invited, that you don't know but that there must be a reason and might not have anything to do with him. If he wants to find out, as they are friends and they do play together at school, he could perfectly ask the other child and based on the response you can then know for sure what really happened.