I would put him back on the food he was on, add some canned food as well (1-2 tablespoons) and mix well, to coat all of the kibble. You can also moisten the food as well to see if he will eat. I never heard of the food you put him on before.
If you are worried about dehydration as well, you can do a 50/50 of water and pedialyte.
# "Breed not a savage dog, nor permit a loose stairway." - Talmud
# "In my day, we didn't have dogs or cats. All I had was Silver Beauty, my beloved paper clip." - Jennifer Hart, Arlington