K.K.
We wanted a positioner but never ended up getting one- so I can't testify to the safety or anything- but I can say that they say once they can pick up their head, the danger of suffocation is less- but I can give you a suggestion on getting him broken of it.
He did eventually fall asleep after you put him down without the positioner, so I'd say to just stick to your guns. Don't go in, don't give him the positioner either. Just let him be and he will soothe himself to sleep. I would also be sure he isn't having gas pains or reflux pains and that's why he feels most comfortable with the positioner. It's probably not the issue but it can't hurt to make sure.