I'd find something very simple.
California Natural has some options (both grain inclusive and grain free) that are VERY simple. They have a puppy formula as well, so you are still getting him enough protein/fat.
We feed Acana (Lamb & Apple and Duck & Pear) which are simple, single source protein foods with only one grain. He does great on them, but also did good on their grain-free variety. Acana is made by the same company as Orijen, but yeah, Orijen can be a little too rich for some dogs (and it's very easy to over-feed).
I'd eliminate the canned Evangers if it were me (but I don't really like Evangers too much as a company, since they lied about ingredients in the past).
You could even try Royal Canin. It may be a *bit* more similar to the Puppy Chow, but definitely much higher quality, IMO and more "middle of the road" in terms of kibble.
But yeah, agree that I wouldn't do a ton of switching, but I would get him off the puppy chow, and get his belly back in order. Maybe even some plain chicken & rice to calm the belly (unless you don't want him tasting the chicken - he may never eat dog food plain again, LOL)
Btw, welcome to YT from another MD'er!