Was he falling asleep on his own before you had the baby? If yes, this could be some regression. Is he in a crib or a bed? Is he getting out on his own or just screaming for you to come back?
I like SuperNanny's approach where you return the child to bed every time they get out of bed. The first time you tell them very sweetly, "No it's time for bed. I love you!" and give them another kiss and/or hug. The second time you are more firm: "No, back to bed." with no kiss or hug. After that you just keep returning them to bed without saying anything or having any other interaction. Some kids will keep this up for hours - once on the show one kid got out of bed something like 137 times! But the parents did the same thing every time - stuck him back in the bed without saying anything. They did not show any emotion or say one word. Eventually they give up because they realize that they are not going to get a different response. Many times kids are persistent because they are just trying to break you down and you have to stand firm.
In another method I heard about, you tell the child they need to stay in their bed, but you will be back to check on them in five minutes. They are willing to wait because that doesn't sound very long and they know you will be back. You come back in five minutes, tell them you are just checking on them, and then leave. Let them know you will be back again in ten minutes. If they start to fuss or get out of bed, take them back to bed and tell them again, you will be back to check on them. Then leave. Keep increasing the length of time until they are asleep.
I have not really had to use this on my daughter because she is really good about going to bed and staying there and falling asleep (it helps to have a routine too - like stories, etc.). But a few times she's started crying after she's been in bed, I've told her I will be back to check on her and that's made her happy. Now she will ask me if I will be back to check on her and I always say yes, and she's fine. I will check on her 10 to 15 minutes later, and she's always zonked out.
Good luck and sweet dreams! I hope this helps!