E.B.
Are you actually sure that he is drinking nearly one quart (which is 32 ounces) of water every night? Does he drink from a bottle at night (like, is he falling asleep with the bottle in his mouth and just sucking on it all night)? Does he wake up to get a sippy cup of water? How much do you think he drinks during the day?
I'm thinking that you or another adult must be waking during the night to refill his bottle or cup. So problem #1 is that he's nearly 18 months old and still waking frequently wanting a bottle or a cup all night long. He's not soothing himself to sleep but relying on a habit of drinking a lot of water, if that's the case.
Is he thirsty? Is he crying for water? Problem #2 could be excessive thirst, and if he's not drinking the water just to have something to suck on, then a doctor should be consulted ASAP because a child who is that thirsty can have a medical issue that needs to be addressed.
Also, did a doctor prescribe a multi-vitamin? Do you cook for him or is the chicken and rice from a package, jar, or frozen?
You might ask your child's doctor for a consult with a nutritionist who can advise you on water, vitamins, and a well-balanced meal plan.
But first, something that you can easily do right now, is to start keeping track of the water and the waking. On a piece of paper, write down (using a page a day) how often your child asks for a cup or bottle of water, and how much is actually in that cup. Use a measuring cup to measure the amount of water that your child's drinking cup will hold. Don't just refill the cup or bottle, wait until it's empty and then refill it, so that you know exactly how much he drank. Also keep track of what he ate that day, and when. Write down how often (and for how long) he wakes during the night to drink water. A doctor will find that very useful.