7-Up might still be considered okay to settle an upset stomach, but I would think that the high fructose corn syrup would exacerbate the diarrhea. Regular corn syrup is often given in bottles of formula to help with constipation--I can't imagine how HFCS wouldn't have the same effect.
Unless a child is showing signs of dehydration, plain water is still the best thing for them. Especially if they are able to eat.
If he can keep down food, the BRAT diet will help bulk up his bowels and slow things down a bit. Bananas, Rice, Applesauce, Toast (dry toast, crackers, etc). If solids aren't an option just yet, you can boil rice in water for about 20 minutes, strain, and cool. The rice liquid will have much the same effect. Plain yogurt will also help counteract the diarrhea.
My daughter also likes a little warm tea when she isn't feeling well. Peppermint is soothing and has a nice flavor without needing much (if any) sweetening.
The key is to offer fluids and food frequently but in small amounts. Even if her only takes a sip or bite every 15 minutes, it will be easier on his system.
Good luck...we just did the 24-hours bug and it is never any fun.