Hi there, welcome to BW forums
In my opinion if you continue with this then you will be risking establishing a habit/prop for sleep.
A milk feed just before bed (but not feeding to sleep) is absolutely normal either before or after teeth are brushed but it really ought to be sufficient to have the milk feed and not require additional liquids for hydration at that time of day or to fall to sleep. A night feed is also totally normal at this age although over the next 1 or 2 months would usually be weaned and I see you have already begun that process to move calorie intake to the day - that's fine.
If it is particularly hot where you are or if LO is poorly then offering water at night might be needed but generally this would be a one off or a short period, not a habit every night and not for helping LO to relax to sleep. The thing to look at is if LO is taking water because he needs water or because he needs help to get to sleep. If he needs water he would be fine having that water fully awake, out of cot and 10 mins before sleep, not in the the cot and nodding off whilst taking it.
Often parents of older toddlers and children may leave a sports bottle or cup of water in LOs room in case they need a drink in the night but this is often to reduce call backs when LOs are going through developmental stages, or when they have a cough or something like that...personally I would not encourage or continue a water habit in the night with a baby this young, nor with an older child who was needing to suck/drink water to get to sleep - there are more appropriate ways to learn to self sooth.
If you make sure he is offered water with all solids meals plus his regular milk feeds this should be plenty to stay hydrated. If he is constipated you might want to offer a prune or two to help him or if it is bad seek medical advice and medicine so it does not worsen. There are some helpful tips here:
Constipation and Laxative Foods