How do I teach my 4 month old to put his pacifier back in his mouth at night?

Just last week we moved our son from a binet in our room to his baby bed in his room. We are also trying to phase out all of his nighttime feedings, per the pediatricians suggestion. For the last 3 nights he has awakened twice crying because his binky had fallen out of his mouth. I gave him his binky and he went back to sleep. How can I teach him to put it back in his mouth? Should I be getting him used to sucking his thumb instead?