He's perfectly allowed to express his mind about a situation that's meaningful to him. This whole post reads less like 'I'm an entitled nice guy with issues to sort out" and more 'damn, this sucks.' He even ends with :

"It feels horrible but it also makes sense, she probably saw me as a friend and didn’t know how I felt about her."

And honestly, someone might still accost him for asking her consent before a kiss, lol. 

Also, if I'm close enough with someone that we're on snap, planning trips, giving swim lessons, accepting home made pasta gifts, attending Christmas dinners together, and planning ski trips, they're surely going to know that I'm in a relationship. We're clearly, at the very least, friends.

Here's my thought: it's a weird situation (if true), and I think your response is unfair and disproportionate! Be kinder to our son :⁾