Hi Barbara...welcome to AP

I, too, have just started my overweight cat, Harley, on the new Purina formula for indoor cats. I haven't noticed any change in her litter box habits, though.
I would bet that your cat is having a hard time digesting the greens in this new food, so it may not be the best thing for him.
If it continues, I'd try something else.
I didn't have any success with the weight control (Iams) food for Harley either. She was on it exclusively for the past 6 months and she's still a "Chunky Butt" (according to her 'Daddy")
I love the name Pit...Is Pit an only child? If so, or if you have some way to isolate his feeding, you might have good luck with some of the Science Diet foods. I understand that you feed less because they are digested better.
Harley is one of 5, so someone is always coming into her room to steal a bite of food, so we have to keep kibble down all the time.
Keep us posted with Pit's progress. We're here to help!