I've been wondering that myself, I already had the idea of him living with just his father, so perhaps either his father crossed someone that cursed Lucas to get back at him for it, or Lucas' father was abusive, so he tried to run away from home, but was caught and cursed to have that appearance so he'd be forced to stay. After all, who'd Lucas have to go to when he looks like that? I'm leaning towards the latter of the two.
As for how he'd be allowed to stay alive, well considering he's staying at the church and studying healing, saying he would be a devoted follower in said church could actually work.