I'm really not sure how he and Gwyn would get on - the most I can recall from these two is Merlin warning Arthur "THAT BITCH GON' CHEAT ON YOU" and Arthur being like "LOL WHAT"
But yeah uhm, boarding school. He and Gwyn are the same age, so if their boarding schools had soirees or wtv there's that. I'm not sure about the Irishness of the Sinclair family, though I have pretty vague memories of Joana mentioning it, but yes Hemera & Gwyn.
Alternatively, Horus is part English.
Since he's an Econ major, there's Hades. Probs NYU students take some classes at Columbia and vice-versa. They would be... interesting.
Hermes will just steamroll on over because it's Merlin and Hermes is a troll, so no need to think about that.