1. Vitamin D.
2. Sleep a lot.
3. Eat as much as you can. Real food preferably. If you haven't been eating much then simple sugars etc. are fine if you need the cals.
Obviously, eliminating simple sugars is a good idea... if you can. If you can't eat any food without throwing up then sure it's fine. Juices are probably better than sodas though.. at least they have some redeeming nutritional value via the minerals and vitamins added.
First off what you said is somewhat true., but what you fail to understand is that in order to process this refined sugar(which is present in most pops) you need vitamins and minerals to catalyze and metabolize to harness the energy. Fruits contain the essential and necessary nutrients to carry out the proper break down of the sugar within the fruit. Where as the refined sugar from pop rarely, if ever, contain the proper nutrient quantities(which is why it is called empty calories), and when these nutrients lack your body goes into a different sort of break down of the sugar(slower break down). This reaction results in the formation of Pyruvic acid and other 5 carbon sugars which cause stress to your body. Your body can usually eliminate the pyruvic acid, unless it accumulates in rather excess amounts which then can accumulate in the brain and in the nervous system. This results results in a lower PH, which greatly affects the homeostasis of your body. The consequence of this fluctuation of your biochemistry can be connected to many different complications ranging from physical to mental.
Also, since sources of refined sugar usually lack the proper nutrients needed for the digestion, valuble nutrients will be leached from your body in order to break it down. This loss of nutrients do result in an impaired immune function, since your immune system needs nutrients for functioning. So going back to that kid, drinking pop probably isn't the best idea, since we just established that the excess sugar stresses the body. And when you stress the body your immune system will have to work twice as hard.
This physiology is terrible.
While you do need various vitamins and minerals you should have enough of them present from when you were not sick (unless you've been sick months) to correctly catalyze reactions.
No nutrients are leeched -- all of the sugars travel to the cells that need energy (which is a lot if you're not eating a lot) and will be metabolized inside the cells there.
Secondly, this acid "mechanism" proposed is garbage. There's so many things wrong with it I don't know where to start.
Pyruvic acid is a normal part of carbohydrate metabolism. The acid produced is not from the "acid" part of the equation, but ANY type of nutrient oxidation produces hydrogen ions. These hydrogen ions are USED by the mitochondria to produce ATP. This occurs within ALL cells that produce energy for the body.
To say that acid is bad for the body is really really really really really stupid. And to say that pyruvic acid lowers body pH is even dumber. Net acid load on the body has other things to do with than eating simple sugars...
If he said it's bad for your body based on insulin response and possibly stress or inflammatory reactions then sure. But this is just a terrible reason.. and even the previous sentence for the most part doesn't matter too much when you need calories to help support your body's energy to some extent (and to prevent muscle wasting if you've been sick a while).