Outside the Lines by Caitlin Ricci

Alex has always wanted to help people, so when he finds a scared, homeless child, he doesn’t hesitate to take ‘Socks’ to the shelter where he works, Trinity House.
Over the course of four years, a chance meeting turns into friendship. When Socks turns eighteen, they’re excited, because it means they can move in with Alex—until Alex rejects them, and Socks realizes an affection they thought mutual was only ever one-sided.
Years later, Socks has become Trin, a psychiatrist who specializes in helping children. And they still want desperately to know why Alex rejected them long ago…