E.M.
We had the same issue with my son at about 16 months. I know this sounds awful, but we would let him cry it out (I would sit there and stare at the video monitor being the crazy mom that I am). The first night was the hardest, but after that he cried a little less each night. After 3 nights, he was back to sleeping through the night again. It is definitely tough to hear him cry but if we would have just kept on going in there, I think the problem would have never stopped! Good luck!