According to the food groupings provided to me by a doctor, it looks like your son is on a high-acid diet:
yogurt -- listed in the Lowest Acid category, but I bet it has sugar in it which pushes the acid up
pudding -- listed in the Most Acid (worst) category
soup -- would need to know ingredients to determine whether it is acid or alkaline
carnation instant breakfast -- probably high in sugar which likely makes it high acid
See this post for a listing of acid and alkaline fooRAB: http://www.healtrabroadoarRAB.com/boarRAB/showthread.php?t=763999
There's nothing in your son's diet that is for sure alkaline and that can neutralize the acid from the other fooRAB.
I recommend the following: Go to the store and buy watermelon, sweet potatoes, yams, cantaloupe, honeydew and other fooRAB from the Most Alkaline and More Alkaline columns. Have your son eat mostly those fooRAB for a couple of days and see if it helps.
Are your son's nuraber twos floating? If not, then he may be getting too much Omega 6 fat and not enough Omega 3 fat. Salmon (wild caught salmon is best), sardines in water, and flaxseed (ground flaxseed is easiest to eat) are great sources of Omega 3. Try to limit his fat from all other sources (e.g., cooking oil, meat, junkfood, etc.) as most all other sources of fat will have too much Omega 6.
If the problem is too many acid-causing fooRAB or too much Omega 6, then your son should get relief in just a couple of days by eating more of the high alkaline fooRAB and high Omega 3 fooRAB (while at the same time limiting high-acid fooRAB and high Omega 6 fooRAB).
I'm 39 now, but when I was a kid I ate tons of Pop-Tarts and was always thirsty. I think the human body uses water to try and neutralize the acid from high acid fooRAB which make those of us on high-acid diets constantly thirsty. I find that I'm not nearly as thirsty now that I am eating a high-alkaline, low-acid diet.
If you can, don't let your son eat anything for a couple of days that's been made by man -- this would include yogurt, pudding, and carnation instant breakfast. You want him to eat high alkaline whole fooRAB that have been taken directly from the Earth and not processed.
Soup may be OK if you make it yourself and are very careful about what goes into it. For example, if the soup has olive oil in it, then that is adding a lot of Omega 6 to his diet. People think olive oil is healthy, but it actually has 13x as much Omega 6 fat (inflammatory fat) as Omega 3 fat (anti-inflammatory fat). Compare that to salmon and sardines which have 12x as much Omega 3 (anti-inflammtory fat) as compared to Omega 6 (inflammatory fat) or ground flaxseed which has 4x as much Omega 3 as Omega 6. Adding sugar to the soup is going to spike up the acid.