My DS did started this at that age too. It is frustrating and tiresome. The truth is that he still does it to some extent. This may not be the best way to deal wih it, and it certainly hasn't stopped it, but I have found it easiest to just go and comfort him - or send DH in to do it whenever it's possible. We have tried everything else, but none of it works and then none of us get any sleep. At least with going into comfort him I am then able to get back to sleep myself. Hope you have better luck then me. You could always try sleep school.