K.M.
I did this with my daughter and her pacifier. When we went to visit my sister out of town I made a big production of forgetting it when we got there and then when it was time for her to lay down at night I reminded her that I forgot it and that now that she was a big girl and we were not at home she had to go to sleep without it. It worked and when we got home I thought I had to do someting else, but by that time she forgot about it. You could do that when he goes to grandmas or say that grandma doesn't have any more bottles at her house and then take them out of the house so it's "out of sight, out of mind". My dentist was not happy with the bottle after 12 months of age. So bad for their teeth. Good Luck!