It's on page 113 of Rabbit Nutrition and Natural Healing by Lucile Moore. The references are at the end of each chapter & I couldn't see which one were for this statement. Anyway, they're all so technical it'd take a biologist or chemist to understand them.
The citrates usually are pills. I wonder how they could be crushed up. You might get them in small pieces using his nail clippers & slipping them in something. I think they need acid to dissolve, so you can't dissolve them in water. Maybe they come in chewable or liquid. If they're capsules, you can open them up & use a little at a time.
I assume any acid food would work. Lemons only advantage is not having much sugar. lol A little orange or lemon juice in with the banana water would be good. Actually, it sounds like the start of a good smoothie.
EDIT Go to vitacost.com & enter potassium citrate. The choices include those with magnesium & one is a liquid, raspberry flavored. Among the capsules, look for those that list gelatin as this will be the kind you can open & sprinkle out what you need. Some of the ones called capsules are just capsule shaped. Vitacost has lots of other goodies as well. I like some of the dried fruit [for me].