Pop/soda replacement needed!!

Most of the sweetened drinks at the store that do not contain sugar contain aspartame, splenda or sugar alcohols (xylitol). There are a few with Stevia in the natural food grocery stores like Whole Foods and Sprouts.

Aspartame converts to formaldehyde when it gets to certain temperature and formaldehyde is a carcinogen.
Splenda is a derivitive of chlorine and has an adverse effect on the thymus gland which regulates the immune system.
Xylitol (sugar alcohols) tend to cause gas and diarrhea.
Stevia - FDA originally refused to add Stevia to their list of "Generally Recognized as Safe" (GRAS) food additives because it was shown to have an adverse effect on the liver, but it must've been added in the past year or two because it is everywhere now.
Truvia is a combination of Stevia and Xylitol.
Sweet n' Low (pink packets) do not have any reported adverse effects.
Sugar, honey or agave are good natural products in small amounts.

Carbonation in the sodas has a naturally occuring sodium, which is what might be causing his aches and pains due to the effects of constant swelling, inflammation and water retention.

Green tea is beneficial in that it boosts metabolism and is high in beneficial anti-oxidants. Flavored teas are good, too. Perhaps, he could lower his soda intake and drink more water and tea.

My boyfriend works at Pepsi and they sell Aquafina 10cal. It is a flavored water, but unlike most flavored waters, it is more like juice than just a light flavor.

Or if he likes the carbonation, try carbonated flavored waters.