Thanks Gasp0de!
I now have an urge to write some fan fiction in which Arthur visits the Discworld.
Vimes would have Carrot "keep an eye on that lad, he's going to get himself killed in under half an hour", and he and Carrot would become firm friends thanks to their shared belief in the brilliance of everything.
Vetinari would be fascinated by him but slightly worried that left to his own devices he'd either ruin the city or accidentally take it over.
Granny Weatherwax would be secretly impressed by how unintimidated by her he seems to be, and secretly touched that he keeps accidentally calling her Mum! 