Tea and coffee can vary strongly in regards of the caffeine levels you have in them, depending on temperature of the water you used to brew, how long you brew it, brand of coffee, and so on. While you totally can calculate how much caffeine you'll end up with in your cup if you have the training/knowledge and equipment/willingness to do so, soda with added caffeine is significantly easier than self-brewed beverages, as their content basically never varies.
As for Soda consumption, as @
Endus said, he drinks sugar-free soda, so there is that. Sure, there are healthier alternatives, but since he has says he's been diagnosed I'd assume he at least discussed caffeine-intake with his attending doctor.
If I were to recommend something I'd probably go with self-made lemonade with ginger tea, little or no sugar, and precisely-dosed caffeine in the form of pills, but that's just me.