Since this is your 5 year old with ADHD/ODD I think you need to take the plunge. No matter what you do it will be difficult for him for a short period--there's no avoiding that most of the time for a child who has had a comfort item for so long. You might try replacing the milk with water but most likely that will only drag it out.
Does your child see a dentist regularly and does he/she know of your problem? They usually go ballistic about bottle use at bedtime because even though the teeth may look fine on the surface, decay can form from the sugar in milk or juice and cause permanent damage. I know how hard it is but your kiddo is old enough for you to reason with on this as well as to understand that his friends at school would tease him like crazy if they knew. Boys of his age still use blankies and stuffed animals every night but they don't use bottles. Make sure he knows that when you go for it--he likely won't care, but he needs to know anyway.
You will need help at bedtime for the first few nights to deal with your little one. Do you have someone who could be there?
