Switching foods

My dog just turned 1 at the beginning of the month and my boyfriend accidentally bought him puppy food again last month. I am trying to switch him back to an adult food since he is full grown and a beagle-prone to being overweight and puppy food has a lot of calories.

Yesterday was the first day I added a small amount of the new food into his existing food. When I say small amount, I mean an EXTREMELY small amount. MY dog has a very sensitive stomach and even with that small amount he was waking me up every 2 hours to go outside with very very loose stool.
I think I am going to 1. stick with chicken and rice today and 2. return this dog food and switch to a basic protein and carb dog food.

Does anyone have any other suggestions for how they switch their dog to a different food and what foods work best? Not only is he up all night having stomach issues, but he is still sleeping and its already 11 where he usually wakes me up at 730 so I know he is not feeling well at all :help: