Don't buy soda.
I know this seems simplistic, but really, he's 5. If you decide to keep buying soda, tell him from now on he can choose one soda a day, whenever he wants it that's fine. Otherwise, provide a bunch of really nice juices, gatorade, lemons and limes to make "citrus water" - wow that's good - a glass of ice water, a teaspoon of sugar and a 1/4 lime or lemon squeezed in.
When he's 17 it would be hard to get him to drink what you want him to, but he's only 5 - it's easy.
my mom bought me lemons, limes, oranges and let me squeeze that in my water for some hint of flavor. even today, that's the way i still have to have my water
I'd omit soda altogether--it's bad for their teeth. And it's better to eat fruits than drink fruit juice. I like the suggestion of squeezing citrus fruits into water.
If water was all he had to drink, he would drink it.
you can also try to put just a pinch of koolaid to give it color..maybe he will drink it..thats what i do with my 3 yr. old..But he does like his plain water too....good luck
or what about flavored water by capri..or dasani..those are really good....
Thanx for all the great suggestions... i tried a few of them but i think my husband had the easiest solution.. he brought home this new drink called Wat-aah and my son just loves it.. http://www.drinkwataah.com/ .. but sometimes i do have to add a lemon when he gets picky..