Runaways by Megan Derr
Addison hates the city. Dragged there by his Uncle, his hopes of museums and bookshops and historic monuments quickly turned into nightmares involving balls, dinner parties, and constantly dancing attendance upon his brat cousin. When his cousin falls sick, Addison seizes the rare chance for solitude and flees to the park.
But he is not there long before he is accosted by a stranger, a man like no one Addison has ever encountered, someone who shows him a world he actually enjoys--but it is also a world to which he will never truly belong.