If he is waking up during the night crying, or even i the morning I think it is recommended you pick them up, cuddle them and then put them down once they are happy, around this age they start to understand who mummy and daddy are, but they dont develop the understanding about you coming back till later, so they wake up in fear because they cant see you, and dont understand you will come for them.
If you leave them I believe it can cause them to become more distressed and they begin to believe you wont come back.
As long as you put them back down before they are asleep, and only comfort them , I believe this is how Tracey taught it.