That's awesome!!20 mins later he used the box and thank god, peed a large amount.
You could put him back on the Instinct, maybe transition him with a 50:50 ratio for one meal or so, then the Instinct at 75 to 25 for the other, see how that goes for him.
Then you can gently transition him to other foods, and as you're thinking, keep some of the Tiki Cat chicken in the rotation, maybe just not as frequently.