I think it may be good to have a discussion with him about immediate needs and order as BEING moral issues.
Could you frame it to him that part of everyone getting what they need is sacrificing non-immediate needs and wants so that others are not distracted by his want to get water? If you are able to shift the center of the discussion from him, to others, you are allowing him to see the moral side of waiting to get a drink.
you can also differentiate between emergent needs (someone's hurt or toilet emergency) and non-emergent needs (wanting a drink and being a little hungry).
by doing so, you are helping him develop his sense of justice using others as a lens, in addition to himself.
