One thing you can do is to make him seem like he influenced you first.
You: I saw that you were feeding your dog X, and since I know you know a lot about dogs I thought I would look into feeding my own (your dog’s name) X. My dog has a sensitivity to certain ingredients (you don’t have to say “to crap”) so I cross-checked the ingredients on a great dog food resource called Dog Food Advisor, I’m sure you’ve heard of it. Anyway, I was pretty surprised by what I found so thought I would share it with you to get your opinion. (Hand him a print out)
Good luck.
