You did well and he will most likely get an infraction, so you beat this little loser twice.
I have a good friend that is very good at world pvp and he likes to camp people in to the ground. He selects a target and makes its live miserable.
The rational ones just turn War-mode off, but the entitled ones, just like your "friend", make alts and whisper him. And if he is lucky they whisper him shit like your "friend".
He then reports it to Blizzard and gets the big win - he gets them banned
Just remember to answer people like him with the utmost politness and put them on ignore when they start to harass you, like your "friend":
Firstly, it makes them even more mad so you get more material to get them banned
Secondly, it shows to Blizzard that you are the victim.
You can always quote them a passage from Blizzard's pvp-policy, for example:
"If you do not wish to engage in open world PvP combat in World of Warcraft, disable War Mode."
https://eu.battle.net/support/en/article/10184
Best of luck with getting more losers banned!