S.A.
My second child did that when she was about that age. She was afraid of her closet...so we turned the light on in the closet, put a small nightlight in her room, and then I gave her a little rubber bat to sleep with so she could hit monsters if she saw one. I guess I should not have let her watch Monsters Inc....her older sister loved the movie.
At any rate, that did the trick for a year and half...now she insists on sleeping in the full bed with her sister in her room. Oh well, if they are both happy, I'm happy.
PS The comment that it is common in children that cry it out so early is silly. Five months is old enough to sleep through the night. Most peds will tell you three months. I nursed all my kids and still nursing my son, so I do get up once or twice with him, but I have let him cry it out at times because he is wanting to cuddle every two hours. Mommy is tired and I'm no good as a mommy when I'm tired. It is just a phase and that's all. Not all kids go through it. My first never did and I let her cry it out at about five months or so. My second never did until she watched that silly movie. And we shall see with my son. You didn't do anything wrong letting your son cry it out. But everyone has an opinion right?