I wish I had the answer but I don't - all I can say is we are going through this with DD at the moment. Strangely enough just for her night sleep, for day naps she's fine. Put her to bed at night and she screams the place down - we rock and pat and then lay her in her cot once calm and have to stay by the cot until she's asleep. Hoping it's a phase - for your DS too