Why is the baby in your bed anyway and not in a crib of his own even if he is in your room? I agree to give him a bottle in the middle of the night if you insist on going in to him, but water it down and keep watering it down until you give him only water in the middle of the night and then just leave a water bottle in the corner of the crib so he can take it for himself if he wakes up and wants it.
In addition, maybe you should move your 7 year old out until you get the baby to sleep through the night.