You are acting like many children aren't rude and intentionally disrespectful towards many adults.. He could be a struggling adult. Adults struggle too.. There are some absolutely cruel and insensitive children in this world who thrive off of finding weak spots, vulnerabilities, points of sensitivity and triggers to push in adults.
Adults are people with feelings, emotions, and breaking points as well. Everyone has their breaking points. A child shouldn't have a free " pass" to treat anyone like crap.. Vice versa. Children and some adults must learn that there can be terrible repercussions and consequences for that type of behavior, and that everyone isn't going to bypass it and put up with poor treatment.
Not saying this is the case with this op and situation.. just saying it is in many cases, and it isn't always the adult.
Don't automatically be dismissive of the adults side, and rush in defense of a child. Hear both sides.
We all say things and lash out sometimes and often don't mean it. Sometimes it is misdirected anger. No one has it "together" and composed all of the time.
No matter how much emotional control we have, sometimes it just automatically comes out. We can't take these things back, but we can try to do better.
Op, if you don't think that you can get past this, and won't look at him in the same light...it may be in the best interest for everyone involved to end things.