I was a newborn and Child Portrait Photographer and studio manager for a few years and there are a few things I did to get babies to go to sleep. They also worked on my son. I would lightly blow into his face and humm at a low pitch with his head near my neck. Also it can be a big help to talk to Mom and Dad before and have him eat before (keep a wipe up towel on hand for potential spit up) if they are a formula family make sure that it's warm. Give the tummy a rub for a bit and 9 times out of ten it would put the baby right out. According to my Photographers I had a special gift to do this but with a little practice you will find it works well. Also don't be afraid to move the arms under the chin to pose the baby might move but just keep reposing, it can take a little while but it will help the newborn feel more comfortable and inspire sleepytime as well as Awws from mom and dad.
Douglas Lee Coon