Since Atlas had no place of his own to go to, he agreed to Lilith's suggestions. That seemed to be the only valid suggestion at the moment, and it was definitely better than going to the Kingdom of Wolves. If it was a small town, they could stay there at peace, without worrying about it much.
Moreover, if it was a small town, even if Luna's brother came, it wasn't going to be easy for him to find them.
Atlas didn't believe that he had a long time. Whatever time he had, he wanted to spend in peace, teaching Misella about her new powers. He wanted to spend his last few days at a place where he didn't need to fight anyone, or kill anyone. He didn't want to use his strength which only decreased his life.
As far as the war with the otherworlders was concerned, he decided to leave it on Castiel. He could only help in the planning, but more than that, he didn't think he needed to get involved. Castiel was a more capable warrior after all.