Yes I would see it as cutting out the BT milk but it's ok if his milk increases somewhere else, say at dinner or morning WU, whenever he has a milk drink, equally though I wouldn't be too concerned if his milk didn't increase elsewhere as the milk he is having plus some dairy solids such as cheese will be enough.
If you currently offer 9 oz I would do
night 1 8oz plus bottle of water
night 3 7oz
night 5 6oz
night 7 5oz... and so on
This would take roughly 18 days which is 2.5 wks. Not very long really.
I would also notice the amount he took on any night if the bottle is not drained and off this as the max amount the night after and subsequent nights. So for instance if on night 2 he is offered 8oz but drinks only 6 and leaves 2oz then the following night you only offer 6oz (not 7). This will likely reduce the time by a couple of days. He might even refuse the milk altogether before the 2.5wks is up and just want a sip of water instead.
If you have any questions whilst you do it I'd be happy to hold your hand and answer as best I can along the way