Your vet did you a great disservice by putting your pup on Iams and Eukanuba. A lot of vets sell it - but it's garbage. It's not your fault; you would think vets would be a little concerned about learning something substantive about nutrition other than the information they get from the large manufacturers. The Purina Mighty Dog is pretty bad, but probably not much worse than the Iams/Eukanuba.
Some dogs and puppies need time to adjust to a different food, and your pup will probably need some adjustment time when you switch him to a good food.
The Innova ingredients are a good example of what to look for; if you hold other foods to those standards you won't go wrong.
The two simplest things I tell people to look for is that the ingredients are
organic and
human grade. Those are always a good place to start.
And don't worry, we're not going to yell at you!
Mordy will probably have some more good guidelines for you.
I'm curious, was there anything other than it being hard that led you to think that your pup wasn't old enough for the Canidae?