Unless your pediatrician has suggested a multivitamin, I’d wait. Of course mine are much older so the thoughts might be different now on this.
We started using multivitamins about 4 I think and it was the Flintstone chewables. I would check with your doctor first before you put your kiddo on anything.
Your other question regarding water intake. My son drinks a lot too. He drinks at least 80 oz per day and if he’s working outside that can easily double. He doesn’t have diabetes, but he drinks a lot. He also doesn’t drink anything but water with the exception of milk in the morning at breakfast. No soda, no juices, just water.
If you’re concerned about his diet and trying to supplement with vitamins, that can be a hard thing to do. Vitamins, unless they’re made a certain way, aren’t absorbed and won’t take the place of a good diet.
My son is anti veggies although when he was younger he was less picky. Keep trying different kinds and introducing them to him. Even a couple of bites is fine. I used to cook veggies and purée them and then load spaghetti sauce with them. Cook your rice in chicken bone broth to add extra protein and it gives good flavor.
Sorry to ramble on, but with your two questions, I thought you’re more concerned with his diet and making sure he gets the nutrition he needs.
Another thing I would tell my son is “big boys love carrots or carrots help you see better or whatever. He was very interested from 1.5-3 to be considered a big boy. ❤️🙂