S.G.
Let him fall asleep with it. Then go in and quietly take it out of his mouth and put it beside him incase he wakes later. If he wakes up while you take it out of his mouth, just let him have it. Go in as often as you can as long as he is sleeping and take it out. He will eventually get used to not having it in his mouth as he sleeps. I did the same thing that you did by only letting him have it in his crib. That worked for me too and so did taking it out of his mouth when he falls asleep.
Good luck!