Switching foods too quickly can easily cause stomach upset, especially in young puppies with sensitive systems. Feeding too much can also cause loose stools -- if you were feeding him the same amount of TOTW as you were the Iams, that could have also contributed to bowel issues (the daily feeding recommendations for premium foods are usually less than for economy brands, since premium brand dog foods contain higher quality ingredients and less fillers).
I'd recommend trying again to switch him over to TOTW -- VERY gradually. Start out by replacing just 1/8 of his Iams with TOTW. Do that for a couple days before you bump it up to 1/4 TOTW and 3/4 Iams. As long as he continues to do well and has no loose stools, slightly increase the amount of TOTW every 2 days while decreasing the amount of Iams. Also remember that when you finally get to the point where he's eating 100% TOTW, his portions should be considerably less than it was with the Iams.
How much are you currently feeding him?
- Fawn Female, born 03/23/2001
- White Female, born 03/27/2007