It depends how long he has been with you I suppose. 18 months is still young but I'd want to put a plan in place with the parents by the time he's 2. And the parents have to be doing what you are doing too.

If he's new ne needs the support system of his blankie, but if he's been with you for a while you can start to phase it out. One of my grandsons is completely addicted to his blankie so the first step we took was to make him put it in a place in the room where it was always visible to him and if he needed, he could go hug it, but he had to leave in in that place. He was allowed to have it for naptime only.

I hate to tell you this but he's 5 and he still needs it for a hug when he's really upset and to sleep, but he must keep it on his bed at home at all times now.