Thank you both for your replies!
Darn squirrels! We live in a pretty dense city, but even in the parks/wooded areas there don't seem to be any oaks or other plant/tree goodies he could get a hold of.
And I agree, not overly impressed with Royal Canin ingredient list and a bit suspicious of the vet $ motives.
The diarrhea has since resolved (for now), and the only pattern I can identify is when we are out of town (once) or going to my parents house with him (they live 30 min away with a great backyard - a luxury we don't have). My guess is the back and forth and new experiences, though to some degree these visits are becoming routine and it hasn't happened every time.
I guess I was more concerned about the strained nature of his regular stool and whether the food was responsible for that, but everything seems to suggest that's just how he digests. He does get plenty of fiber (regular pumpkin and he is also a fan of veggie scraps, always under my feet when the cutting board is out).
I will keep an eye out for if he gets picky at any point. For now I think I'll stick with TotW which he seems to really enjoy, and I've added the probiotic the vet gave me.