Oliver won't eat dry food

Hi all! My cat Oliver is a year and a half old, and a few days ago he started acting weird. Was really lethargic, wasn't eating or drinking as far as I could tell, and he would get in his litter box and scratch around a bit, but I'm not sure if he was actually eliminating. I free feed him (and my other cat, Prince) a gentle formula of dry food.
Anyway, after about two days of Oliver acting this way, I finally got him to eat some wet food and a little bit of tuna, even a few treats. He perked up and has been acting normal ever since; he's got energy, is alert and playful, but still has no interest in his dry food.
What should I do??? I know I can't keep feeding him a diet of straight tuna, because of magnesium and heavy metal levels.

I also think it's worth mentioning that I've poked and prodded him to see if he cries out in pain, to check for things like kidney stones; and no luck, Chuck. He loves it when I massage his belly and "nether regions"..
He hasn't cried out in any pain or anything. I've checked his mouth/gums; everything seems fine!

Don't feed dry food. Cats are obligate carnivores and need a high protein meat based diet. Kibble is cheap, loaded with carbs, and causes chronic dehydration and has been linked to urinary issue, kidney failure, and obesity. Look at moving all your cats onto a wet or raw food diet. Any time you have a cat acting off you need to take them to the vet to be evaluated. 

Dry food is considered by many knowledgeable vets to be a poor choice for cats for many reasons. Good quality canned (without wheat, corn, or soy) is far better or raw if you want to go that route.

http://www.blakkatz.com/dryfood.html